MDEV-31939 Adaptive flush recommendation ignores dirty ratio and checkpoint age

buf_flush_page_cleaner(): Pass pct_lwm=srv_max_dirty_pages_pct_lwm
(innodb_max_dirty_pages_pct_lwm) to
page_cleaner_flush_pages_recommendation() unless the dirty page ratio
of the buffer pool is below that. Starting with
commit d4265fbd we used to always
pass pct_lwm=0.0, which was not intended.

Reviewed by: Vladislav Vaintroub

storage/innobase/buf/buf0flu.cc

@@ -2379,12 +2379,13 @@ static os_thread_ret_t DECLARE_THREAD(buf_flush_page_cleaner)(void*)
       else
       {
       maybe_unemployed:
-        const bool below{dirty_pct < pct_lwm};
+        if (dirty_pct < pct_lwm)
+        {
           pct_lwm= 0.0;
-        if (below)
           goto possibly_unemployed;
         }
       }
+    }
     else if (dirty_pct < srv_max_buf_pool_modified_pct)
     possibly_unemployed:
       if (!lsn_limit && !soft_lsn_limit && !af_needed_for_redo(oldest_lsn))