In commit f360d88a, 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: f360d88a ("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>