selftests: ublk: setup ring with IORING_SETUP_SINGLE_ISSUER/IORING_SETUP_DEFER_TASKRUN

It is observed that this way is more efficient for fast nvme backing
file.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-8-ming.lei@redhat.com


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