io_uring: open code io_cqring_overflow_flush()

There is only one caller of io_cqring_overflow_flush(), open code it

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a1fecd56d9dba923ed8d4d159727fa939d3baa2a.1712708261.git.asml.silence@gmail.com


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