ublk: call io_uring_cmd_to_pdu to get uring_cmd pdu

Call io_uring_cmd_to_pdu() to get uring_cmd pdu, and one big benefit
is the automatic pdu size build check.

Suggested-by: Uday Shankar <ushankar@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250327095123.179113-6-ming.lei@redhat.com


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