IB/core: Use qp->usecnt to track multicast attach/detach

Just as we don't allow PDs, CQs, etc. to be destroyed if there are QPs
that are attached to them, don't let a QP be destroyed if there are
multicast group(s) attached to it.  Use the existing usecnt field of
struct ib_qp which was added by commit 0e0ec7e ("RDMA/core: Export
ib_open_qp() to share XRC TGT QPs") to track this.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
diff --git a/include/rdma/ib_verbs.h b/include/rdma/ib_verbs.h
index c3cca5a..c84d101 100644
--- a/include/rdma/ib_verbs.h
+++ b/include/rdma/ib_verbs.h
@@ -964,7 +964,7 @@
 	struct ib_srq	       *srq;
 	struct ib_xrcd	       *xrcd; /* XRC TGT QPs only */
 	struct list_head	xrcd_list;
-	atomic_t		usecnt; /* count times opened */
+	atomic_t		usecnt; /* count times opened, mcast attaches */
 	struct list_head	open_list;
 	struct ib_qp           *real_qp;
 	struct ib_uobject      *uobject;