memcg: remove warning when CONFIG_DEBUG_VM=n

mm/memcontrol.c:318: warning: `mem_cgroup_is_obsolete' defined but not used

[akpm@linux-foundation.org: simplify as suggested by Balbir]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-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

@@ -932,7 +932,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	if (unlikely(!mem))
 		return 0;
 
-	VM_BUG_ON(mem_cgroup_is_obsolete(mem));
+	VM_BUG_ON(!mem || mem_cgroup_is_obsolete(mem));
 
 	while (1) {
 		int ret;