IPoIB: Retry failed send-only multicast group joins

When a send-only multicast group join fails, mcast->query must be set
to NULL.  Otherwise, IPoIB will never retry the join and the multicast
group will never be reachable.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>