IB/mlx5: Enable "block multicast loopback" for kernel consumers

IB/mlx5: Enable "block multicast loopback" for kernel consumers

In commit 8d4753de41ae, we advertise blocking multicast loopback to both
kernel and userspace consumers, but don't allow kernel consumers (e.g IPoIB)
to use it with their UD QPs.  Fix that.

Fixes: 8d4753de41ae ("IB/mlx5: Add block multicast loopback support")
Reported-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>