ublk_drv: don't forward io commands in reserve order

ublk_drv: don't forward io commands in reserve order

Either ublk_can_use_task_work() is true or not, io commands are
forwarded to ublk server in reverse order, since llist_add() is
always to add one element to the head of the list.

Even though block layer doesn't guarantee request dispatch order,
requests should be sent to hardware in the sequence order generated
from io scheduler, which usually considers the request's LBA, and
order is often important for HDD.

So forward io commands in the sequence made from io scheduler by
aligning task work with current io_uring command's batch handling,
and it has been observed that both can get similar performance data
if IORING_SETUP_COOP_TASKRUN is set from ublk server.

Reported-by: Andreas Hindborg <andreas.hindborg@wdc.com>
Cc: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221121155645.396272-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>