datapath-backport: vlan: introduce __vlan_insert_tag helper which does not free skb

There's a need for helper which inserts vlan tag but does not free the
skb in case of an error.

Upstream: 15255a43 ("vlan: introduce __vlan_insert_tag helper which does not free skb")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath-backport: move make_writable helper into common code

note that skb_make_writable already exists in net/netfilter/core.c
but does something slightly different.

Upstream: e219512 ("net: move make_writable helper into common code")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath-backport: move vlan pop/push functions into common code

So it can be used from out of openvswitch code.
Did couple of cosmetic changes on the way, namely variable naming and
adding support for 8021AD proto.

Upstream: 93515d53 ("net: move vlan pop/push functions into common code")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath-backport: introduce rtnl ops stub

This stub now allows userspace to see IFLA_INFO_KIND for ovs master and
IFLA_INFO_SLAVE_KIND for slave.

Upstream: 5b9e7e16 ("openvswitch: introduce rtnl ops stub")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath-backport: replace remaining users of arch_fast_hash with jhash

This patch effectively reverts commit 500f80872645 ("net: ovs: use CRC32
accelerated flow hash if available"), and other remaining arch_fast_hash()
users such as from nfsd via commit 6282cd565553 ("NFSD: Don't hand out
delegations for 30 seconds after recalling them.") where it has been used
as a hash function for bloom filtering.

While we think that these users are actually not much of concern, it has
been requested to remove the arch_fast_hash() library bits that arose
from [1] entirely as per recent discussion [2]. The main argument is that
using it as a hash may introduce bias due to its linearity (see avalanche
criterion) and thus makes it less clear (though we tried to document that)
when this security/performance trade-off is actually acceptable for a
general purpose library function.

Lets therefore avoid any further confusion on this matter and remove it to
prevent any future accidental misuse of it. For the time being, this is
going to make hashing of flow keys a bit more expensive in the ovs case,
but future work could reevaluate a different hashing discipline.

  [1] https://patchwork.ozlabs.org/patch/299369/
  [2] https://patchwork.ozlabs.org/patch/418756/

Upstream: 8754589 ("net: replace remaining users of arch_fast_hash with jhash")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
travis: Update kernel build matrix to include 3.18.x

Also updates all older kernel builds to use latest stable release.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>