rtnetlink: fix FDB size computation

Add missing NDA_VLAN attribute's size.

Fixes: 1e53d5bb

 ("net: Pass VLAN ID to rtnl_fdb_notify.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>