ublk_drv: do not add a re-issued request aborted previously to ioucmd's task_work

In ublk_queue_rq(), Assume current request is a re-issued request aborted
previously in monitor_work because the ubq_daemon(ioucmd's task) is
PF_EXITING. For this request, we cannot call
io_uring_cmd_complete_in_task() anymore because at that moment io_uring
context may be freed in case that no inflight ioucmd exists. Otherwise,
we may cause null-deref in ctx->fallback_work.

Add a check on UBLK_IO_FLAG_ABORTED to prevent the above situation. This
check is safe and makes sense.

Note: monitor_work sets UBLK_IO_FLAG_ABORTED and ends this request
(releasing the tag). Then the request is restarted(allocating the tag)
and we are here. Since releasing/allocating a tag implies smp_mb(),
finding UBLK_IO_FLAG_ABORTED guarantees that here is a re-issued request
aborted previously.

Suggested-by: Ming Lei <[email protected]>
Signed-off-by: ZiyangZhang <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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