vxlan: Have vxlan_fdb_replace() save original rdst value

To enable rollbacks after vetoed FDB updates, extend vxlan_fdb_replace()
to take an additional argument where it should store the original values
of a modified rdst. Update the sole caller.

The following patch will make use of the saved value.

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