selftests: ublk: pass 'ublk_thread *' to more common helpers

Pass 'ublk_thread *' to more common helpers, then we can avoid to store
this reference into 'struct ublk_io'.

Prepare for supporting to handle IO via different task context.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-14-ming.lei@redhat.com


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