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: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220815023633.259825-3-ZiyangZhang@linux.alibaba.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>