mm: memcontrol: move PageMemcgKmem to the scope of CONFIG_MEMCG_KMEM

The page only can be marked as kmem when CONFIG_MEMCG_KMEM is enabled.
So move PageMemcgKmem() to the scope of the CONFIG_MEMCG_KMEM.

As a bonus, on !CONFIG_MEMCG_KMEM build some code can be compiled out.

Link: https://lkml.kernel.org/r/20210319163821.20704-8-songmuchun@bytedance.comSigned-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -523,6 +523,7 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
 	return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
 
+#ifdef CONFIG_MEMCG_KMEM
 /*
  * PageMemcgKmem - check if the page has MemcgKmem flag set
  * @page: a pointer to the page struct
@@ -537,7 +538,6 @@ static inline bool PageMemcgKmem(struct page *page)
 	return page->memcg_data & MEMCG_DATA_KMEM;
 }
 
-#ifdef CONFIG_MEMCG_KMEM
 /*
  * page_objcgs - get the object cgroups vector associated with a page
  * @page: a pointer to the page struct
@@ -579,6 +579,11 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
 }
 
 #else
+static inline bool PageMemcgKmem(struct page *page)
+{
+	return false;
+}
+
 static inline struct obj_cgroup **page_objcgs(struct page *page)
 {
 	return NULL;