Page cleaner sleeps indefinitely despite innodb_max_dirty_pages_pct_lwm being exceeded

MDEV-24278 improved the page cleaner so that it will no longer wake up once per second on an idle server. However, with innodb_adaptive_flushing (the default) the function page_cleaner_flush_pages_recommendation() could initially return 0 even if there is work to do. The page cleaner should only remain sleeping indefinitely as long as the sharp innodb_max_dirty_pages_pct_lwm limit is not exceeded.

Based on a source code comment, it appears that an initial intention of innodb_max_dirty_pages_pct_lwm=0.0 (the default value) was to disable something. That ceased to hold in MDEV-23855 and MDEV-24278: the value is a pure threshold; the page cleaner will not perform any work unless the threshold is exceeded.