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.

af_get_pct_for_dirty(): Remove. Based on a comment here, 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: the value is a pure threshold; the page cleaner will not
perform any work unless the threshold is exceeded.

page_cleaner_flush_pages_recommendation(): Add the parameter dirty_blocks
to ensure that buf_pool.flush_list will eventually be emptied.