memcg: fix build warning and avoid checking for mem != null again and again

Fix build warning, "mem_cgroup_is_obsolete defined but not used" when
CONFIG_DEBUG_VM is not set.  Also avoid checking for !mem again and again.
Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -314,14 +314,6 @@ static struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
 	return mem;
 }
 
-static bool mem_cgroup_is_obsolete(struct mem_cgroup *mem)
-{
-	if (!mem)
-		return true;
-	return css_is_removed(&mem->css);
-}
-
-
 /*
  * Call callback function against all cgroup under hierarchy tree.
  */
@@ -932,7 +924,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	if (unlikely(!mem))
 		return 0;
 
-	VM_BUG_ON(!mem || mem_cgroup_is_obsolete(mem));
+	VM_BUG_ON(css_is_removed(&mem->css));
 
 	while (1) {
 		int ret;