memcg: move mem_cgroup_is_root upwards

kmem code uses this function and it is better to not use forward
declarations for static inline functions as some (older) compilers don't
like it:

gcc version 4.3.4 [gcc-4_3-branch revision 152973] (SUSE Linux)

  mm/memcontrol.c:421: warning: `mem_cgroup_is_root' declared inline after being called
  mm/memcontrol.c:421: warning: previous declaration of `mem_cgroup_is_root' was here

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -412,10 +412,14 @@ struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
 	return container_of(s, struct mem_cgroup, css);
 }
 
+static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
+{
+	return (memcg == root_mem_cgroup);
+}
+
 /* Writing them here to avoid exposing memcg's inner layout */
 #if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
 
-static bool mem_cgroup_is_root(struct mem_cgroup *memcg);
 void sock_update_memcg(struct sock *sk)
 {
 	if (mem_cgroup_sockets_enabled) {
@@ -1011,11 +1015,6 @@ void mem_cgroup_iter_break(struct mem_cgroup *root,
 	     iter != NULL;				\
 	     iter = mem_cgroup_iter(NULL, iter, NULL))
 
-static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
-{
-	return (memcg == root_mem_cgroup);
-}
-
 void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx)
 {
 	struct mem_cgroup *memcg;