memcg: remove redundant check in move_task()

It's guaranteed by cgroup that old_cgrp != cgrp.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.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 fba566c..7056c3b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1168,9 +1168,6 @@
 	mem = mem_cgroup_from_cont(cont);
 	old_mem = mem_cgroup_from_cont(old_cont);
 
-	if (mem == old_mem)
-		goto out;
-
 	/*
 	 * Only thread group leaders are allowed to migrate, the mm_struct is
 	 * in effect owned by the leader