sched: fix minor bug in yield

- fix a minor bug in yield (seen for CONFIG_FAIR_GROUP_SCHED),
  group scheduling would skew when yield was called.

Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched_fair.c

@@ -725,7 +725,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int sleep)
  */
 static void yield_task_fair(struct rq *rq)
 {
-	struct cfs_rq *cfs_rq = &rq->cfs;
+	struct cfs_rq *cfs_rq = task_cfs_rq(rq->curr);
 	struct rb_node **link = &cfs_rq->tasks_timeline.rb_node;
 	struct sched_entity *rightmost, *se = &rq->curr->se;
 	struct rb_node *parent;