ublk_drv: update comment for __ublk_fail_req()

Since __ublk_rq_task_work always fails requests immediately during
exiting, __ublk_fail_req() is only called from abort context during
exiting. So lock is unnecessary.
Signed-off-by: NZiyangZhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220815023633.259825-3-ZiyangZhang@linux.alibaba.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -605,8 +605,9 @@ static void ublk_complete_rq(struct request *req)
 }
 
 /*
- * __ublk_fail_req() may be called from abort context or ->ubq_daemon
- * context during exiting, so lock is required.
+ * Since __ublk_rq_task_work always fails requests immediately during
+ * exiting, __ublk_fail_req() is only called from abort context during
+ * exiting. So lock is unnecessary.
  *
  * Also aborting may not be started yet, keep in mind that one failed
  * request may be issued by block layer again.