RDMA/iwcm: Don't touch cmid after dropping reference

The function cm_work_handler() cannot touch the cm_id after it derefs
it, because it might be freed on another concurrent thread.  If there
are more work items queued for this cm_id, then we know there must be
more references because they are added when the work items are queued.
So in the while loop inside cm_work_handler(), after derefing, if the
queue is empty, then exit the function.  Otherwise we know it's safe
to re-acquire the lock.
Signed-off-by: NSteve Wise <swise@opengridcomputing.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>

drivers/infiniband/core/iwcm.c

@@ -878,6 +878,8 @@ static void cm_work_handler(struct work_struct *_work)
 			}
 			return;
 		}
+		if (empty)
+			return;
 		spin_lock_irqsave(&cm_id_priv->lock, flags);
 	}
 	spin_unlock_irqrestore(&cm_id_priv->lock, flags);