io_uring: share completion list w/ per-op space

Calling io_req_complete(req) means that the request is done, and there
is nothing left but to clean it up. That also means that per-op data
after that should not be used, so we're free to reuse it in completion
path, e.g. to store overflow_list as done in this patch.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>