selftests: ublk: add helper ublk_handle_uring_cmd() for handle ublk command

Add helper ublk_handle_uring_cmd() for handling ublk command, and make
ublk_handle_cqe() more readable.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-17-ming.lei@redhat.com


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