io_uring: normilise naming for fill_cqe*

Restore consistency in __io_fill_cqe* like helpers, always honouring
"io_" prefix and adding "req" when we're passing in a request.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bd016ff5c1a4f74687828069d2619d8a65e0c6d7.1647481208.git.asml.silence@gmail.com


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