io_uring: fix mismatched finish_wait() calls in io_uring_cancel_files()

The prepare_to_wait() and finish_wait() calls in io_uring_cancel_files()
are mismatched. Currently I don't see any issues related this bug, just
find it by learning codes.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>