mm: memory.numa_stat: fix file permission

Commit 406eb0c9ba76 ("memcg: add memory.numastat api for numa
statistics") adds memory.numa_stat file for memory cgroup.  But the file
permissions are wrong.

  [kamezawa@bluextal linux-2.6]$ ls -l /cgroup/memory/A/memory.numa_stat
  ---------- 1 root root 0 Jun  9 18:36 /cgroup/memory/A/memory.numa_stat

This patch fixes the permission as

  [root@bluextal kamezawa]# ls -l /cgroup/memory/A/memory.numa_stat
  -r--r--r-- 1 root root 0 Jun 10 16:49 /cgroup/memory/A/memory.numa_stat

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e37c44d..02a7947 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4640,6 +4640,7 @@
 	{
 		.name = "numa_stat",
 		.open = mem_control_numa_stat_open,
+		.mode = S_IRUGO,
 	},
 #endif
 };