ZEN: Tune mgLRU to protect cache used in the last second

Although not identical to the le9 patches that protect a byte-amount of
cache through tunables, multigenerational LRU now supports protecting
cache accessed in the last X milliseconds.

In torvalds#218, Yu recommends starting with 1000ms and tuning as needed.  This
looks like a safe default and turning on this feature should help users
that don't know they need it.

mm/vmscan.c

@@ -4333,7 +4333,11 @@ static int get_swappiness(struct lruvec *lruvec)
 	return swappiness;
 }
 
+#ifdef CONFIG_ZEN_INTERACTIVE
+static DEFINE_RATELIMIT_STATE(lru_gen_min_ttl, 1000, 1);
+#else
 static DEFINE_RATELIMIT_STATE(lru_gen_min_ttl, 0, 1);
+#endif
 
 static unsigned long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
 				    int swappiness)