mm/memcontrol.c: fix "integer as NULL pointer" sparse warning

mm/memcontrol.c:2548:32: warning: Using plain integer as NULL pointer
Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2545,7 +2545,7 @@ static int mem_cgroup_force_empty_list(struct mem_cgroup *mem,
 		pc = list_entry(list->prev, struct page_cgroup, lru);
 		if (busy == pc) {
 			list_move(&pc->lru, list);
-			busy = 0;
+			busy = NULL;
 			spin_unlock_irqrestore(&zone->lru_lock, flags);
 			continue;
 		}