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

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>