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/drivers/infiniband/core/verbs.c b/drivers/infiniband/core/verbs.c
index 575b780..c5b94d6 100644
--- a/drivers/infiniband/core/verbs.c
+++ b/drivers/infiniband/core/verbs.c
@@ -1183,23 +1183,33 @@
 
 int ib_attach_mcast(struct ib_qp *qp, union ib_gid *gid, u16 lid)
 {
+	int ret;
+
 	if (!qp->device->attach_mcast)
 		return -ENOSYS;
 	if (gid->raw[0] != 0xff || qp->qp_type != IB_QPT_UD)
 		return -EINVAL;
 
-	return qp->device->attach_mcast(qp, gid, lid);
+	ret = qp->device->attach_mcast(qp, gid, lid);
+	if (!ret)
+		atomic_inc(&qp->usecnt);
+	return ret;
 }
 EXPORT_SYMBOL(ib_attach_mcast);
 
 int ib_detach_mcast(struct ib_qp *qp, union ib_gid *gid, u16 lid)
 {
+	int ret;
+
 	if (!qp->device->detach_mcast)
 		return -ENOSYS;
 	if (gid->raw[0] != 0xff || qp->qp_type != IB_QPT_UD)
 		return -EINVAL;
 
-	return qp->device->detach_mcast(qp, gid, lid);
+	ret = qp->device->detach_mcast(qp, gid, lid);
+	if (!ret)
+		atomic_dec(&qp->usecnt);
+	return ret;
 }
 EXPORT_SYMBOL(ib_detach_mcast);