userspace: add support for pop_eth and push_eth actions

These actions will allow L2->L3 and L3->L2 switching, and are supposed
to be added to flows installed in the datapath transparently by
ovs-vswitchd.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>
userspace: add layer 3 flow and switching support

This commit relaxes the assumption that all packets have an Ethernet
header, and adds support for layer 3 flows.  For each packet received on
the Linux kernel datapath the l2 and l3 members of struct ofpbuf are
intialized appropriately, and some functions now expect this (notable
flow_extract()), in order to differentiate between layer 2 and layer 3
packets.  struct flow has now a new 'noeth' member, because we cannot
assume that a flow has no Ethernet header when eth_src and eth_dst are
0.  For layer 3 packets, the protocol type is still stored in the
eth_type member.

Switching L2->L3 and L3->L2 are both implemented by adding the pop_eth
and push_eth actions respectively when a transition is detected.  The
push_eth action puts 0s on both source and destination MACs.  These
addresses can be modified with mod_dl_dst and mod_dl_src actions.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>
datapath: add layer 3 flow/port support

Implementation of the pop_eth and push_eth actions in the kernel, also
layer 3 flow support.  Jesse Gross provided feedback on a previous
version of this RFC patch, all of those comments are resolved here.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>