IB/multicast: Check ib_find_pkey() return value

This patch avoids that Coverity complains about not checking the
ib_find_pkey() return value.

Fixes: commit 547af76521b3 ("IB/multicast: Report errors on multicast groups if P_key changes")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/multicast.c

@@ -518,8 +518,11 @@ static void join_handler(int status, struct ib_sa_mcmember_rec *rec,
 		process_join_error(group, status);
 	else {
 		int mgids_changed, is_mgid0;
-		ib_find_pkey(group->port->dev->device, group->port->port_num,
-			     be16_to_cpu(rec->pkey), &pkey_index);
+
+		if (ib_find_pkey(group->port->dev->device,
+				 group->port->port_num, be16_to_cpu(rec->pkey),
+				 &pkey_index))
+			pkey_index = MCAST_INVALID_PKEY_INDEX;
 
 		spin_lock_irq(&group->port->lock);
 		if (group->state == MCAST_BUSY &&