selftests: bridge_mdb: Add MDB bulk deletion test

Add test cases to verify the behavior of the MDB bulk deletion
functionality in the bridge driver.

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