ip: retain local interface address adjacency

ip: retain local interface address adjacency

Type: feature

ip4_local_check_src() was overwriting vnet buffer opaque data
on the adjacency for packets with "local" (dpo-receive) destination
addresses.

Retain the dpo receive index in vnet_buffer()->adj_index[VLIB_TX].
This can allow a graph node to distinguish the interface where the
destination address is configured from the interface where the
packet was received. This can be useful in correctly handling
packets that have been sent to an address configured on a loopback
interface.

Change-Id: I52a942e85b5302b338a2d0404a37c5ea1a99e89f
Signed-off-by: Matthew Smith <mgsmith@netgate.com>