memcg: null dereference on allocation failure

The original code had a null dereference if alloc_percpu() failed.  This
was introduced in commit 711d3d2c ("memcg: cpu hotplug aware percpu
count updates")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4208,15 +4208,17 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 
 	memset(mem, 0, size);
 	mem->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
-	if (!mem->stat) {
+	if (!mem->stat)
+		goto out_free;
+	spin_lock_init(&mem->pcp_counter_lock);
+	return mem;
+
+out_free:
 	if (size < PAGE_SIZE)
 		kfree(mem);
 	else
 		vfree(mem);
-		mem = NULL;
-	}
-	spin_lock_init(&mem->pcp_counter_lock);
-	return mem;
+	return NULL;
 }
 
 /*