Simplify adjacency rewrite code

Simplify adjacency rewrite code

Using memcpy instead of complex specific copy logic. This simplify
the implementation and also improve perf slightly.
Also move adjacency data from tail to head of buffer, which improves
cache locality (header and data share the same cacheline)
Finally, fix VxLAN which used to workaround vnet_rewrite logic.

Change-Id: I770ddad9846f7ee505aa99ad417e6a61d5cbbefa
Signed-off-by: Benoît Ganne <bganne@cisco.com>