io_uring: explicitly keep a CQE in io_kiocb

We already have req->{result,user_data,cflags}, which mimic struct
io_uring_cqe and are intended to store CQE data. Combine them into a
struct io_uring_cqe field.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e1efe65d5005cd6a9ec3440767eb15a9fa9351cf.1649771823.git.asml.silence@gmail.com
[axboe: add mirror cqe to cater to fd union]
Signed-off-by: Jens Axboe <axboe@kernel.dk>