IB/uverbs: track multicast group membership for userspace QPs

uverbs needs to track which multicast groups is each qp
attached to, in order to properly detach when cleanup
is performed on device file close.

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>