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 raspberrypi#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

@@ -4138,7 +4138,11 @@ static bool age_lruvec(struct lruvec *lruvec, struct scan_control *sc,
 }
 
 /* Protect the working set accessed within the last N milliseconds. */
+#ifdef CONFIG_ZEN_INTERACTIVE
+static unsigned long lru_gen_min_ttl = 1000;
+#else
 static unsigned long lru_gen_min_ttl __read_mostly;
+#endif
 
 static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
 {