A small refactoring for io_req_add_compl_list() deduplicating some code.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f0a5272b45efe4ffc41cb79b99784e39c699aade.1648209006.git.asml.silence@gmail.com


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