io_uring: finish IOU_OK -> IOU_COMPLETE transition

IOU_COMPLETE is more descriptive, in that it explicitly says that the
return value means "please post a completion for this request". This
patch completes the transition from IOU_OK to IOU_COMPLETE, replacing
existing IOU_OK users.

This is a purely mechanical change.

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