io_uring: put the flag changing code in the same spot

Both iocb_flags() and kiocb_set_rw_flags() are inline and modify
kiocb->ki_flags. Place them close, so they can be potentially better
optimised.

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