vxlan: Split vxlan_fdb_update() in two

In order to make it easier to implement rollbacks after FDB update
vetoing, separate the FDB update code to two parts: one that deals with
updates of existing FDB entries, and one that creates new entries.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>