ublk: kill queuing request by task_work_add

ublk: kill queuing request by task_work_add

task_work_add() is used from early ublk development stage for handling
request in batch. However, since commit 7d4a93176e01 ("ublk_drv: don't
forward io commands in reserve order"), we can get similar batch
processing with io_uring_cmd_complete_in_task(), and similar performance
data is observed between task_work_add() and
io_uring_cmd_complete_in_task().

Meantime we can kill one fast code path, which is actually seldom used
given it is common to build ublk driver as module.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230519065030.351216-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>