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>