rtnetlink: bridge: Invoke MDB bulk deletion when needed

Invoke the new MDB bulk deletion device operation when the 'NLM_F_BULK'
flag is set in the netlink message header.

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>