IPoIB: Fix address handle refcounting for multicast groups

Multiple ipoib_neigh structures on mcast->neigh_list may point to the
same ah.  This means that ipoib_mcast_free() can't just make a list of
ah structs to free, since this might end up trying to add the same ah
to the list more than once.  Handle this in ipoib_multicast.c in the
same way as it is handled in ipoib_main.c for struct ipoib_path.

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>