io_uring: consolidate overflow flushing

Consolidate __io_cqring_overflow_flush and io_cqring_overflow_kill()
into a single function as it once was, it's easier to work with it this
way.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/986b42c35e76a6be7aa0cdcda0a236a2222da3a7.1712708261.git.asml.silence@gmail.com


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