ublk_drv: add ublk_queue_cmd() for cleanup

Add helper of ublk_queue_cmd() so that both ublk_queue_rq()
and ublk_handle_need_get_data() can reuse this helper.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221029010432.598367-5-ming.lei@redhat.com


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