io_uring: refactor io_flush_cached_reqs()

Emphasize that return value of io_flush_cached_reqs() depends on number
of requests in the cache. It looks nicer and might help tools from
false-negative analyses.

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