sched: Consolidate the name of root_task_group and init_task_group

root_task_group is the leftover of USER_SCHED, now it's always
same to init_task_group.
But as Mike suggested, root_task_group is maybe the suitable name
to keep for a tree.
So in this patch:
  init_task_group      --> root_task_group
  init_task_group_load --> root_task_group_load
  INIT_TASK_GROUP_LOAD --> ROOT_TASK_GROUP_LOAD

Suggested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110107071736.GA32635@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 777cd01..341acbbc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2511,7 +2511,7 @@
 
 #ifdef CONFIG_CGROUP_SCHED
 
-extern struct task_group init_task_group;
+extern struct task_group root_task_group;
 
 extern struct task_group *sched_create_group(struct task_group *parent);
 extern void sched_destroy_group(struct task_group *tg);