selftests: ublk: pass 'ublk_thread *' to ->queue_io() and ->tgt_io_done()

'struct thread' is task local structure, and the related code will become
more readable if we pass it via parameter.

Meantime pass 'ublk_thread *' to ublk_io_alloc_sqes(), and this way is
natural since we use per-thread io_uring for handling IO.

More importantly it helps much for removing the current ubq_daemon or
per-io-task limit.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-13-ming.lei@redhat.com


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