io_uring: split io_req_complete_post() and add a helper

Split io_req_complete_post(), this is a prep for the next patch.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211207093951.247840-5-haoxu@linux.alibaba.com


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