memcg: remove redundant check in move_task()

It's guaranteed by cgroup that old_cgrp != cgrp.
Signed-off-by: NLi 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: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1168,9 +1168,6 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
 	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