bridge: Add br_fdb_clear_offload()

When a driver unoffloads all FDB entries en bloc, it's inefficient to
send the switchdev notification one by one. Add a helper that unsets the
offload flag on FDB entries on a given bridge port and VLAN.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>