odp: Allow VLAN actions after MPLS actions

OpenFlow 1.1 and 1.2, and 1.3 differ in their handling of MPLS actions in the
presence of VLAN tags. To allow correct behaviour to be committed in
each situation, this patch adds a second round of VLAN tag action
handling to commit_odp_actions(), which occurs after MPLS actions. This
is implemented with two new fields in struct xlate_ctx,
'post_mpls_vlan_tci' and 'next_vlan_tci'.

Any modifications to the VLAN TCI by actions are made to 'next_vlan_tci'.
Initially 'next_vlan_tci' points to 'xin->flow->vlan_tci' so that VLAN
actions are committed before any MPLS actions.  When an MPLS action is
composed 'next_vlan_tci' is updated to point to 'post_mpls_vlan_tci'. This
causes subsequent VLAN actions to be committed after MPLS actions.

Both before and after this patch MPLS LSEs are pushed onto a packet after
any VLAN tags that may be present. This is the behaviour described in
OpenFlow 1.1 and 1.2. OpenFlow 1.3 specifies that MPLS LSEs should be
pushed onto a packet before any VLAN tags that are present. Support
for this will be added by a subsequent patch that makes use of
the infrastructure added by this patch.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>

---

v2.47
* No change

v2.46 [Simon Horman]
* Updated changelog as it had become out of date

v2.45 [Simon Horman]
* As pointed out by Ben Pfaff and Joe Stringer
  + Update VLAN handling in the presence of MPLS push

    Previously the test for committing ODP VLAN actions after MPLS actions
    was that the VLAN TCI differed before and after the MPLS push action.
    This results in a false negative in some cases including if a VLAN tag
    is pushed after the MPLS push action in such a way that it duplicates
    the VLAN tag present before the MPLS push action.

    This is resolved by ensuring the VLAN_CFI bit of the value used to
    track the desired VLAN TCI after an MPLS push action is zero unless
    VLAN actions should be committed after MPLS actions.

  + Update tests to use ovs-ofctl monitor "-m" to allow full inspection of
    MPLS LSE and VLAN tags present in packets.

v2.44 [Simon Horman]
* Rebase for the following changes:
  f47ea02 ("Set datapath mask bits when setting a flow field.")
  7fdb60a ("Add support for write-actions")
  7358063 ("odp-util: Elaborate the comment for odp_flow_format() function.")
* Correct final_vlan_tci and next_vlan_tci initialisation in xlate_actions__()

v2.43 [Simon Horman]
* As suggested by Ben Pfaff
  Move vlan state into struct xlate_ctx

  1. Add final_vlan_tci member to struct xlate_ctx instead of vlan_tci member
     struct xlate_xin.  This seems to be a better palace for it as it does
     not need to be accessible from the caller.

  2. Move local vlan_tci variable of do_xlate_actions() to be the
     next_vlan_tci member of strict xlate_ctx.  This allows for it to work
     across resubmit actions and goto table instructions.

v2.42
* No change

v2.41 [Joe Stringer via Simon Horman]
* Rework comments to make things a little clearer

v2.40 [Simon Horman]
* Rebase for removal of mpls_depth from struct flow

v2.38 - v2.39
* No change

v2.37
* Rebase

v2.36
* No change

v2.5 [Joe Stringer]
* First post
lib: Support pushing of MPLS LSE before or after VLAN tag

This patch modifies the push_mpls behaviour to allow
pushing of an MPLS LSE either before any VLAN tag that may be present.

Pushing the MPLS LSE before any VLAN tag that is present is the
behaviour specified in OpenFlow 1.3.

Pushing the MPLS LSE after the any VLAN tag that is present is the
behaviour specified in OpenFlow 1.1 and 1.2. This is the only behaviour
that was supported prior to this patch.

When an push_mpls action has been inserted using OpenFlow 1.2 or earlier
the behaviour of pushing the MPLS LSE before any VLAN tag that may be
present is implemented by by inserting VLAN actions around the MPLS push
action during odp translation; Pop VLAN tags before committing MPLS
actions, and push the expected VLAN tag afterwards.

The trigger condition for the two different behaviours is the value of the
mpls_before_vlan field of struct ofpact_push_mpls.  This field is set when
parsing OpenFlow actions.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>

---

v2.46 - v2.47
* No change

v2.45 [Simon Horman]
* Rebased for updates to VLAN handling in the presence of MPLS
* Update tests to use ovs-ofctl monitor "-m" to allow full
  inspection of MPLS LSE and VLAN tags present in packets.

v2.44
* No change

v2.43 [Simon Horman]
* Trivial rebase as the result of replacing 'mpls_before_vlan' field
  of struct ofpact_push_mpls with a 'position' field.

v2.42
* No change

v2.41 [Simon Horman]
* Use 'mpls_before_vlan' field of struct ofpact_push_mpls.
* Reword changelog to describe the difference in behaviour between
  different Open Flow versions.

v2.40 [Simon Horman]
* Trivial rebase for removal of set_ethertype()

v2.36 - v2.39
* No change

v2.35 [Joe Stringer]
* First post
datapath: Break out deacceleration portion of vlan_push

Break out deacceleration portion of vlan_push into vlan_put
so that it may be re-used by mpls_push.

For both vlan_push and mpls_push if there is an accelerated VLAN tag
present then it should be deaccelerated, adding it to the data of
the skb, before the new tag is added.

Signed-off-by: Simon Horman <horms@verge.net.au>

---

v2.41 - v2.47
* No change

v2.40
* As suggested by Jesse Gross
  + Simplify vlan_push by returning an error code
    rather than an error code encoded as a struct xkb_buff *

v2.39
* First post
datapath: Add basic MPLS support to kernel

Allow datapath to recognize and extract MPLS labels into flow keys
and execute actions which push, pop, and set labels on packets.

Based heavily on work by Leo Alterman, Ravi K, Isaku Yamahata and Joe Stringer.

Cc: Ravi K <rkerur@gmail.com>
Cc: Leo Alterman <lalterman@nicira.com>
Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Cc: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>

---

v2.47
* Rebase for HAVE_RHEL_OVS_HOOK and OVS_KEY_ATTR_TCP_FLAGS

v2.43 - v2.46
* No change

v2.42
* Rebase for:
  + 0585f7a ("datapath: Simplify mega-flow APIs.")
  + a097c0b ("datapath: Restructure datapath.c and flow.c")
* As suggested by Jesse Gross
  + Take into account that push_mpls() will have freed the skb on error
  + Remove dubious !eth_p_mpls(skb->protocol) condition from push_mpls
    The !eth_p_mpls(skb->protocol) condition on setting inner_protocol
    has no effect. Its motivation was to ensure that inner_protocol was
    only set the first time that mpls_push occured. However this is already
    ensured by the !ovs_skb_get_inner_protocol(skb) condition.
  + Return -EINVAL instead of -ENOMEM from pop_mpls() if the skb is too short
  + Do not add @inner_protocol to kernel doc for struct ovs_skb_cb.
    The patch no longer adds an inner_protocol member to struct ovs_skb_cb
  + Do not add and set otherwise unsued inner_protocol variable in
    rpl_dev_queue_xmit()
* As suggested by Pravin Shelar
  + Implement compatibility code in existing rpl_skb_gso_segment
    rather than introducing to use rpl___skb_gso_segment

v2.41
* No change

v2.40
* Rebase for:
  + New dev_queue_xmit compat code
  + Updated put_vlan()
* As suggested by Jesse Gross
  + Remove bogus mac_len update from push_mpls()
  + Slightly simplify push_mpls() by using eth_hdr()
  + Remove dubious condition !eth_p_mpls(inner_protocol) on
    an skb being considered to be MPLS in netdev_send()
  + Only use compatibility code for MPLS GSO segmentation on kernels
    older than 3.11
  + Revamp setting of inner_protocol
    1. Do not unconditionally set inner_protocol to the value of
       skb->protocol in ovs_execute_actions().
    2. Initialise inner_protocol it to zero only if compatibility code is in
       use. In the case where compatibility code is not in use it will either
       be zero due since the allocation of the skb or some other value set
       by some other user.
    3. Conditionally set the inner_protocol in push_mpls() to the value of
       skb->protocol when entering push_mpls(). The condition is that
       inner_protocol is zero and the value of skb->protocol is not an MPLS
       ethernet type.
    - This new scheme:
      + Pushes logic to set inner_protocol closer to the case where it is
	needed.
      + Avoids over-writing values set by other users.
* As suggested by Pravin Shelar
  + Only set and restore skb->protocol in rpl___skb_gso_segment() in the
    case of MPLS
  + Add inner_protocol field to struct ovs_gso_cb instead of ovs_skb_cb.
    This moves compatibility code closer to where it is used
    and creates fewer differences with mainline.
* Update comment on mac_len updates in datapath/actions.c
* Remove HAVE_INNER_PROCOTOL and instead just check
  against kernel version 3.11 directly.
  HAVE_INNER_PROCOTOL is a hang-over from work done prior
  to the merge of inner_protocol into the kernel.
* Remove dubious condition !eth_p_mpls(inner_protocol) on
  using inner_protocol as the type in rpl_skb_network_protocol()
* Do not update type of features in rpl_dev_queue_xmit.
  Though arguably correct this is not an inherent part of
  the changes made by this patch.
* Use skb_cow_head() in push_mpls()
  + Call skb_cow_head(skb, MPLS_HLEN) instead of
    make_writable(skb, skb->mac_len) to ensure that there is enough head
    room to push an MPLS LSE regardless of whether the skb is cloned or not.
  + This is consistent with the behaviour of rpl__vlan_put_tag().
  + This is a fix for crashes reported when performing mpls_push
    with headroom less than 4. This problem was introduced in v3.36.
* Skip popping in mpls_pop if the skb is too short to contain an MPLS LSE

v2.39
* Rebase for removal of vlan, checksum and skb->mark compat code

v2.38
* Rebase for SCTP support
* Refactor validate_tp_port() to iterate over eth_types rather
  than open-coding the loop. With the addition of SCTP this logic
  is now used three times.

v2.36 - v2.37
* Rebase

* Do not add set_ethertype() to datapath/actions.c.
  As this patch has evolved this function had devolved into
  to sets of functionality wrapped into a single function with
  only one line of common code. Refactor things to simply
  open-code setting the ether type in the two locations where
  set_ethertype() was previously used. The aim here is to improve
  readability.

* Update setting skb->ethertype after mpls push and pop.
  - In the case of push_mpls it should be set unconditionally
    as in v2.35 the behaviour of this function to always push
    an MPLS LSE before any VLAN tags.
  - In the case of mpls_pop eth_p_mpls(skb->protocol) is a better
    test than skb->protocol != htons(ETH_P_8021Q) as it will give the
    correct behaviour in the presence of other VLAN ethernet types,
    for example 0x88a8 which is used by 802.1ad. Moreover, it seems
    correct to update the ethernet type if it was previously set
    according to the top-most MPLS LSE.

* Deaccelerate VLANs when pushing MPLS tags the
  - Since v2.35 MPLS push will insert an MPLS LSE before any VLAN tags.
    This means that if an accelerated tag is present it should be
    deaccelerated to ensure it ends up in the correct position.

* Update skb->mac_len in push_mpls() so that it will be correct
  when used by a subsequent call to pop_mpls().

  As things stand I do not believe this is strictly necessary as
  ovs-vswitchd will not send a pop MPLS action after a push MPLS action.
  However, I have added this in order to code more defensively as I believe
  that if such a sequence did occur it would be rather unobvious why
  it didn't work.

* Do not add skb_cow_head() call in push_mpls().
  It is unnecessary as there is a make_writable() call.
  This change was also made in v2.30 but some how the
  code regressed between then and v2.35.

v2.35
* Rebase
* Move MPLS constants to mpls.h
* Push MPLS tags after ethernet, before VLAN tags
  - This is consistent with the OpenFlow 1.3 specification
  - Compatibility with OpenFlow 1.2 and earlier versions
    may be provided by ovs-vswitchd.
* Correct GSO behaviour in the presence of MPLS but absence of VLANs

v2.34
* Rebase for megaflow changes

v2.33
* Ensure that inner_protocol is always set to to the current
  skb->protocol value in ovs_execute_actions(). This ensures
  it is set to the correct value in the absence of a push_mpls action.
  Also remove setting of inner_protocol in push_mpls() as
  it duplicates the code now in ovs_execute_actions().
* Call __skb_gso_segment() instead of skb_gso_segment() from
  rpl___skb_gso_segment() in the case that HAVE___SKB_GSO_SEGMENT is set.
  This was a typo.

v2.32
* As suggested by Jesse Gross
  - Use int instead of size_t in validate_and_copy_actions__().
  - Fix crazy edit mess in pop_mpls() action comment
  - Move eth_p_mpls() into mpls.h
  - Refactor skb_gso_segment MPLS handling into rpl_skb_gso_segment
    Address Jesse's comments regarding this code:
    "Can we push this completely into the skb_gso_segment() compatibility
     code? It's both nicer and may make the interactions with the vlan code
     less confusing."
  - Move GSO compatibility code into linux/compat/gso.*
  - Set skb->protocol on mpls_push and mpls_pop in the presence
    of an offloaded VLAN.

v2.31
* As suggested by Jesse Gross
  - There is no need to make mac_header_end inline as it is not in a header file
  - Remove dubious if (*skb_ethertype == ethertype) optimisation from
    set_ethertype
  - Only set skb->protocol in push_mpls() or pop_mpls() for non-VLAN packets
  - Use MAX_ETH_TYPES instead of SAMPLE_ACTION_DEPTH for array size
    of types in struct eth_types. This corrects a typo/thinko.
  - Correct eth type tracking logic such that start isn't advanced
    when entering a sample action, ensuring that all possibly types
    are checked when verifying nested actions.
* Define HAVE_INNER_PROTOCOL based on kernel version.
  inner_protocol has been merged into net-next and should appear in
  v3.11 so there is no longer a need for a acinclude.m4 test to check for it.
* Add MPLS GSO compatibility code.
  This is for use on kernels that do not have MPLS GSO support.
  Thanks to Joe Stringer for his work on this.

v2.30
* As suggested by Jesse Gross
  - Use skb_cow_head in push_mpls to ensure there is sufficient headroom for
    skb_push
  - Call make_writable with skb->mac_len instead of skb->mac_len + MPLS_HLEN
    in push_mpls as only the first skb->mac_len bytes of existing packet data
    are modified.
  - Rename skb_mac_header_end as mac_header_end, this seems
    to be a more appropriate name for a local function.
  - Remove OVS_CSUM_COMPLETE code from set_ethertype().
    Inside OVS the ethernet header is not covered by OVS_CSUM_COMPLETE.
  - Use __skb_pull() instead of skb_pull() in pop_mpls()
  - Decrement and decrement skb->mac_len when poping and pushing VLAN tags.
    Previously mac_len was reset, but this would result in forgetting
    the MPLS label stack.
  - Remove spurious comment from before do_execute_actions().
  - Move OVS_KEY_ATTR_MPLS attribute to its final, upstreamable, location.
  - Correct ethertype check for OVS_ACTION_ATTR_POP_MPLS case in
    validate_and_copy_actions() to check for MPLS ethertypes rather than
    ETH_P_IP.
  - Rewrite tracking of eth types used to verify actions in the presence
    of sample actions. There is a large comment above struct eth_types
    describing the new implementation.

v2.29
* Break include/ and lib/ portions of the patch out into a
  separate patch "datapath: Add basic MPLS support to kernel"
* Update for new MPLS GSO scheme
  - skb->protocol is set to the new ethertype of the packet
    on MPLS push and pop
  - When pushing the first MPLS LSE onto a previously non-MPLS
    packet set skb->inner_protocol to the original ethertype.
  - skb->inner_protocol may be used by the network stack
    for GSO of the inner-packet.
* Drop const from ethertype parameter of set_ethertype.
  This appears to be a legacy of this parameter being a pointer.
* Pass the ethertype patrameter of pop_mpls as a value rather
  than a pointer.

v2.28
* Kernel Datapath changes as suggested by Jarno Rajahalme
  + Correct the logic introduced in v2.27 to set the network_header
    to after the MPLS label stack in the case of an MPLS packet.
    - Increment stack_len offset so that label stacks of depth greater
      than two do not cause an infinite loop.
    - Correct offset passed to check_header to include skb->mac len

v2.27
* Kernel Datapath changes as suggested by Jarno Rajahalme and Jesse Gross:
  + Previously the mac_len and network_header of an skb corresponded
    to the end of the L2 header.  To support GSO, just before transmission,
    do_output, with the results as follows:

    Input: non-MPLS skb: Output: network header and mac_len correspond
                         to the beginning of the L3 headers
    Input: MPLS:         Output: network header and mac_len correspond to the
                         end of the L2 headers.

    This is somewhat confusing.

  + The new scheme is as follows:
    - The mac_len always corresponds to the end of the L2 header.
    - The network header always corresponds to the beginning of the
      L3 header.

  + Note that in the case of MPLS output the end of the L2 headers and the
  beginning of the L3 headers will differ.

* Remove unused declaration of skb_cb_mpls_stack()

v2.26
* Rebase on master
* Kernel Datapath changes as suggested by Jarno Rajahalme
  - Use skb_network_header() instead of skb_mac_header() to locate
    the ethertype to set in set_ethertype() as the latter will
    be wrong in the presence of VLAN tags. This resolves
    a regression introduced in v2.24.
  - Enhance comment in do_output()
  - do_execute_actions(): Do not alter mpls_stack_depth if
    a MPLS push or pop action fail. This is achieved by altering
    mpls_stack_depth at the end of push_mpls() and pop_mpls().

v2.25
* Rebase on master
* Pass big-endian value as the last argument of eth_types_set() in
  validate_and_copy_actions__()
* Use revised GSO support as provided by the patch series
  "[PATCH 0/2] Small Modifications to GSO to allow segmentation of MPLS"
  - Set skb->mac_len to the length of the l2 header + MPLS stack length
  - Update skb->network_header accordingly
  - Set skb->encapsulated_features

v2.24
* Use skb_mac_header() in set_ethertype()
* Set skb->encapsulation in set_ethertype() to support MPLS GSO.
  Also add a note about the other requirements for MPLS GSO.
  MPLS GSO support will be posted as a patch net-next (Linux mainline)
  "MPLS: Add limited GSO support"
* Do not add ETH_TYPE_MIN, it is no longer used

v2.23
* As suggested by Jesse Gross:
  - Verify the current ethernet type when validating sample actions
    both for the taken and not-taken path if the sample action.
  - Document that the OVS_KEY_ATTR_MPLS attribute accepts a list of
    struct ovs_key_mpls but that an implementation may restrict
    the length it accepts.
  - Restrict the array length of the OVS_KEY_ATTR_MPLS to one.
    + Don't add ovs_flow_verify_key_len as it was added to
      handle attributes whose values are arrays but there are
      no attributes with values that are arrays (of length greater than one).

v2.22
* As suggested by Jesse Gross:
  - Fix sparse warning in validate_and_copy_actions()
    I have no idea why sparse doesn't show this up this on my system.
  - Remove call to skb_cow_head() from push_mpls() as it
    is already covered by a call to make_writable()
  - Check (key_type > OVS_KEY_ATTR_MAX) in ovs_flow_verify_key_len()
  - Disallow set actions on l2.5+ data and MPLS push and pop actions
    after an MPLS pop action as there is no verification that the packet
    is actually of the new ethernet type. This may later be supported
    using recirculation or by other means.
  - Do not add spurious debuging message to ovs_flow_cmd_new_or_set()

v2.21
* As suggested by Jesse Gross:
  - Verify that l3 and l4 actions always always occur prior to
    a push_mpls action and use the network header pointer of an skb
    to track the top of the MPLS stack. This avoids adding an l2_size
    element to the skb callback.

v2.20
* As suggested by Jesse Gross:
  - Do not add ovs_dp_ioctl_hook
    + This appears to be garbage from a rebase
  - Do not add skb_cb_set_l2_size. Instead set OVS_CB(skb)->l2_size
    in ovs_flow_extract().
  - Do not free skb on error in push_mpls(), it is freed in the caller
  - Call skb_reset_mac_len() in pop_mpls() and push_mpls()
  - Update checksums in pop_mpls(), push_mpls() and set_mpls().
  - Rename skb_cb_mpls_bos() as skb_cb_mpls_stack().
    It returns the top not the bottom of the stack.
  - Track the current eth_type in validate_and_copy_actions
    which is initially the eth_type of the flow and may be modified
    by push_mpls and pop_mpls actions. Use this to correctly validate
    mpls_set actions. This is to allow mpls_set actions to be applied
    to a non-MPLS frame after an mpls_push action (although ovs-vswitchd
    doesn't currently do that).
    Also:
    + Remove the check of the eth_type in set_mpls() as the new validation
      scheme should ensure it cannot be incorrect.
    + Use the current eth_type to validate mpls_pop actions and remove
      the eth_type check from pop_mpls().
  - Move OVS_KEY_ATTR_MPLS to non-upstream group in ovs_key_lens
  - Remove unnecessary memset of mpls_key in ovs_flow_to_nlattrs()
  - Make a union of the mpls and ip elements of struct sw_flow_key.
    Currently the code stops parsing after an MPLS header so it is
    not possible for the ip and mpls elements to be used simultaneously
    and some space can be saved by using a union.
  - Allow an array of MPLS key attributes
    + Currently all but the first element is ignored
    + User-space needs to be updated to accept more than one element,
      currently it will treat their presence as an error
  - Do not update network header in ovs_flow_extract() for after parsing
    the MPLS stack as it is never used because no l3+ processing
    occurs on MPLS frames.
  - Allow multiple MPLS entries in a match by allowing the OVS_KEY_ATTR_MPLS
    to be an array of struct ovs_key_mpls with at least one entry.
    Currently only one entry is used which is byte-for-byte compatible with
    the previous scheme of having OVS_KEY_ATTR_MPLS as a struct
    ovs_key_mpls.
* Make skb writable in pop_mpls(), push_mpls() and set_mpls().

v2.18 - v2.19
* No change

v2.17
* As suggested by Ben Pfaff
  - Use consistent terminology for MPLS.
    + Consistently refer to the MPLS component of a packet as the
      MPLS label stack and entries in the stack as MPLS label stack entries
      (LSE).  An MPLS label is a component of an MPLS label stack entry.
      The other components are the traffic class (TC), time to live (TTL)
      and bottom of stack (BoS) bit.
  - Rename compose_.*mpls_ functions as execute_.*mpls_

v2.16
* No change

v2.15
* As suggested by Ben Pfaff
  - Use OVS_ACTION_SET to set OVS_KEY_ATTR_MPLS instead of
    OVS_ACTION_ATTR_SET_MPLS

v2.14
* Remove include/linux/openvswitch.h portion which added add
  new key and action attributes. This
  now present in "User-Space MPLS actions and matches"
  which is now a dependency of this patch

v2.13
* As suggested by Jarno Rajahalme
  - Rename mpls_bos element of ovs_skb_cb as l2_size as it is set and used
    regardless of if an MPLS stack is present or not. Update the name of
    helper functions and documentation accordingly.
  - Ensure that skb_cb_mpls_bos() never returns NULL
* Correct endieness in eth_p_mpls()

v2.12
* Update skb and network header on MPLS extraction in ovs_flow_extract()
* Use NULL in skb_cb_mpls_bos()
* Add eth_p_mpls helper

v2.10 - v2.11
* No change

v2.9
* datapath: Always update the mpls bos if  vlan_pop is successful

  Regardless of the details of how a successful
  vlan_pop is achieved, the mpls bos needs to be updated.

  Without this fix it has been observed that the following
  results in malformed packets

v2.8
* No change

v2.7
* Rebase

v2.6
* As suggested by Yamahata-san
  - Do not guard against label == 0 for
    OVS_ACTION_ATTR_SET_MPLS in validate_actions().
    A label of 0 is valid
  - Remove comment stupulating that if
    the top_label element of struct sw_flow_key is 0 then
    there is no MPLS label. An MPLS label of 0 is valid
    and the correct check if ethertype is
    ntohs(ETH_TYPE_MPLS) or ntohs(ETH_TYPE_MPLS_MCAST)

v2.4 - v2.5
* No change

v2.3
* s/mpls_stack/mpls_bos/
  This is in keeping with the naming used in the OpenFlow 1.3 specification

v2.2
* Call skb_reset_mac_header() in skb_cb_set_mpls_stack()
  eth_hdr(skb) is non-NULL when called in skb_cb_set_mpls_stack().
* Add a call to skb_cb_set_mpls_stack() in ovs_packet_cmd_execute().
  I apologise that I have mislaid my notes on this but
  it avoids a kernel panic. I can investigate again if necessary.
* Use struct ovs_action_push_mpls instead of
  __be16 to decode OVS_ACTION_ATTR_PUSH_MPLS in validate_actions(). This is
  consistent with the data format for the attribute.
* Indentation fix in skb_cb_mpls_stack(). [cosmetic]

v2.1
* Manual rebase