io_uring: inline io_cqring_mark_overflow()

There is only one user of it and the name is misleading, get rid of it
by inlining. By the way make overflow_flush's return value deduction
simpler.

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