In commit a635c40648 (MDEV-27774)
the logic for waking up the buf_pool_page_cleaner() was revised.
Because ReleaseBlocks was being passed by value to CIterate,
only copies of the data member rb.modified were ever modified
during the iteration.
We must use Iterate instead of CIterate to pass the object by
reference.
Thanks to Krunal Bauskar for pointing this out.