MDEV-26356 fixup: Adjust innodb_max_purge_lag_wait

innodb_max_purge_lag_wait_update(): To align with
purge_coordinator_state::refresh(), we must trigger a page flush batch
if the last log checkpoint is too old.

This was caught in a hang of the test innodb_gis.rtree_compress.

storage/innobase/handler/ha_innodb.cc

@@ -228,6 +228,14 @@ static void innodb_max_purge_lag_wait_update(THD *thd, st_mysql_sys_var *,
   {
     if (thd_kill_level(thd))
       break;
+    /* Adjust for purge_coordinator_state::refresh() */
+    mysql_mutex_lock(&log_sys.mutex);
+    const lsn_t last= log_sys.last_checkpoint_lsn,
+      max_age= log_sys.max_checkpoint_age;
+    mysql_mutex_unlock(&log_sys.mutex);
+    const lsn_t lsn= log_sys.get_lsn();
+    if ((lsn - last) / 4 >= max_age / 5)
+      buf_flush_ahead(last + max_age / 5, false);
     srv_wake_purge_thread_if_not_active();
     std::this_thread::sleep_for(std::chrono::milliseconds(100));
   }