selftests: ublk: remove `tag` parameter of ->tgt_io_done()

The `tag` parameter can be figured out from cqe->user_data, and that is
also the only way to get the info, so remove `tag` parameter, and
let target code retrieve it from cqe->user_data.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-12-ming.lei@redhat.com


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