IB/mlx5: Add block multicast loopback support

Add support for the block multicast loopback QP creation flag along
the proper firmware API for that.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>