net: switchdev: support static FDB addresses

This patch adds a is_static boolean to the switchdev_obj_fdb structure,
in order to set the ndm_state to either NUD_NOARP or NUD_REACHABLE.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>