ublk: cleanup ublk_ctrl_uring_cmd

Move all per-command work into the per-command ublk_ctrl_* helpers
instead of being split over those, ublk_ctrl_cmd_validate, and the main
ublk_ctrl_uring_cmd handler.  To facilitate that, the old
ublk_ctrl_stop_dev function that just contained two function calls is
folded into both callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-6-hch@lst.de


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