ublk: comment on ubq->canceling handling in ublk_queue_rq()

In ublk_queue_rq(), ubq->canceling has to be handled after ->fail_io and
->force_abort are dealt with, otherwise the request may not be failed
when deleting disk.

Add comment on this usage.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250327095123.179113-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1310,6 +1310,11 @@ static blk_status_t ublk_queue_rq(struct blk_mq_hw_ctx *hctx,
 	if (ublk_nosrv_should_queue_io(ubq) && unlikely(ubq->force_abort))
 		return BLK_STS_IOERR;
 
+	/*
+	 * ->canceling has to be handled after ->force_abort and ->fail_io
+	 * is dealt with, otherwise this request may not be failed in case
+	 * of recovery, and cause hang when deleting disk
+	 */
 	if (unlikely(ubq->canceling)) {
 		__ublk_abort_rq(ubq, rq);
 		return BLK_STS_OK;