io_uring: remove struct io_completion

We keep struct io_completion only as a temporal storage of cflags, Place
it in io_kiocb, it's cleaner, removes extra bits and even might be used
for future optimisations.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5299bd5c223204065464bd87a515d0e405316086.1633373302.git.asml.silence@gmail.com


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