nvmet-rdma: Fix possible NULL deref when handling rdma cm events

When we initiate queue teardown sequence we call rdma_destroy_qp
which clears cm_id->qp, afterwards we call rdma_destroy_id, but
we might see a rdma_cm event in between with a cleared cm_id->qp
so watch out for that and silently ignore the event because this
means that the queue teardown sequence is in progress.
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>

drivers/nvme/target/rdma.c

@@ -1352,6 +1352,12 @@ static int nvmet_rdma_cm_handler(struct rdma_cm_id *cm_id,
 	case RDMA_CM_EVENT_ADDR_CHANGE:
 	case RDMA_CM_EVENT_DISCONNECTED:
 	case RDMA_CM_EVENT_TIMEWAIT_EXIT:
+		/*
+		 * We might end up here when we already freed the qp
+		 * which means queue release sequence is in progress,
+		 * so don't get in the way...
+		 */
+		if (queue)
 			nvmet_rdma_queue_disconnect(queue);
 		break;
 	case RDMA_CM_EVENT_DEVICE_REMOVAL: