selftests: ublk: add generic_01 for verifying sequential IO order

block layer, ublk and io_uring might re-order IO in the past

- plug

- queue ublk io command via task work

Add one test for verifying if sequential WRITE IO is dispatched in order.

- null target is taken, so we can just observe io order from
`tracepoint:block:block_rq_complete` which represents the dispatch order

- WRITE IO is taken because READ may come from system-wide utility

Cc: Uday Shankar <ushankar@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-2-ming.lei@redhat.com


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