io_uring: fix lost cqe->flags

Don't forget to fill cqe->flags properly in io_submit_flush_completions()

Fixes: a1d7c393 ("io_uring: enable READ/WRITE to use deferred completions")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>