This is a 10.5 backport of 10.6
commit d4265fbd.

page_cleaner_flush_pages_recommendation(): If dirty_pct is
between innodb_max_dirty_pages_pct_lwm
and innodb_max_dirty_pages_pct,
scale the effort relative to how close we are to
innodb_max_dirty_pages_pct.

The previous formula was missing a multiplication by 100.