rtnetlink: bridge: Enable MDB bulk deletion

Now that both the common code as well as individual drivers support MDB
bulk deletion, allow user space to make such requests.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>