memcgroup: return negative error code in mem_cgroup_create()

Cgroup requires the subsystem to return negative error code on error in the
create method.

Signed-off-by: Li Zefan <[e-mail]>
Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Acked-by: Balbir Singh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 		mem = kzalloc(sizeof(struct mem_cgroup), GFP_KERNEL);
 
 	if (mem == NULL)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	res_counter_init(&mem->res);
 
···
 		free_mem_cgroup_per_zone_info(mem, node);
 	if (cont->parent != NULL)
 		kfree(mem);
-	return NULL;
+	return ERR_PTR(-ENOMEM);
 }
 
 static void mem_cgroup_pre_destroy(struct cgroup_subsys *ss,