Busy server prefers LRU flushing over flush list flushing too strongly

With the current 5.7 version of the fix for https://bugs.launchpad.net/percona-server/+bug/1234562 / http://bugs.mysql.com/bug.php?id=70500, the main cleaner coordinator thread loop looks like

... /* Flush LRU: */
  pc_flush(0, 0, &n_processed_lru, &n_flushed_list);
  ut_ad(n_flushed_list == 0);
... /* LRU iteration took too long */
  if (ut_time_ms() > next_loop_time)
   ret_sleep = OS_SYNC_TIME_EXCEEDED;
... /* Sync flush? */
  else if (ret_sleep != OS_SYNC_TIME_EXCEEDED
      && srv_flush_sync
      && buf_flush_sync_lsn > 0) {
... /* adaptive flush? */
  } else if (srv_check_activity(last_activity)) {
... /* idle flush? */
  } else if (ret_sleep == OS_SYNC_TIME_EXCEEDED) {
...
  } else {
   /* no activity, but woken up by event */
  }

Thus, if LRU took too long, we won't go into sync flush, even if that is requested, in which case it will cause performance degradation.