linux-cp: use default adjacency

linux-cp: use default adjacency

Type: fix

Outbound packets from the host have an adjacency lookup performed so
buffer metadata can be filled in and output features can be applied.
If no adjacency is found for a packet, it gets dropped. This breaks
DHCP and possibly other things since the DHCP reply to a discover
request is sent to a destination MAC address that is determined by
the contents of the request packet rather than any existing neighbor
table entry.

If adjacency lookup for outbound packets does not find an entry, use
the default adjacency for the interface & address family.

Change-Id: Ia53a4df3a5bad2991768cfe4a84c560b879e656f
Signed-off-by: Matthew Smith <mgsmith@netgate.com>