mm: memcontrol: micro-optimize mem_cgroup_split_huge_fixup()

Don't call lookup_page_cgroup() when memcg is disabled.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3174,7 +3174,7 @@ static inline void memcg_unregister_all_caches(struct mem_cgroup *memcg)
  */
 void mem_cgroup_split_huge_fixup(struct page *head)
 {
-	struct page_cgroup *head_pc = lookup_page_cgroup(head);
+	struct page_cgroup *head_pc;
 	struct page_cgroup *pc;
 	struct mem_cgroup *memcg;
 	int i;
@@ -3182,6 +3182,8 @@ void mem_cgroup_split_huge_fixup(struct page *head)
 	if (mem_cgroup_disabled())
 		return;
 
+	head_pc = lookup_page_cgroup(head);
+
 	memcg = head_pc->mem_cgroup;
 	for (i = 1; i < HPAGE_PMD_NR; i++) {
 		pc = head_pc + i;