commit a9933105 (MDEV-24537)
introduced the regression that the page cleaner will keep sleeping
even if there is work to do.

innodb_max_dirty_pages_pct_update(): Always wake up the page cleaner
on any SET GLOBAL innodb_max_dirty_pages_pct= assignment.

buf_flush_page_cleaner(): If innodb_max_dirty_pages_pct is nonzero,
consult only that parameter when determining whether there is work
to do. Else, consult innodb_max_dirty_pages.