net: add ndo_fdb_del_bulk

Add a new netdev op called ndo_fdb_del_bulk, it will be later used for
driver-specific bulk delete implementation dispatched from rtnetlink. The
first user will be the bridge, we need it to signal to rtnetlink from
the driver that we support bulk delete operation (NLM_F_BULK).

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>