sched: Fix the broken sched_rr_get_interval()

The caller of sched_sliced() should pass se.cfs_rq and se as the
arguments, however in sched_rr_get_interval() we gave it
rq.cfs_rq and se, which made the following computation obviously
wrong.

The change was introduced by commit:

  7703493 sched: fix crash in sys_sched_rr_get_interval()

... 5 years ago, while it had been the correct 'cfs_rq_of' before
the commit. The change seems to be irrelevant to the commit
msg, which was to return a 0 timeslice for tasks that are on an
idle runqueue. So I believe that was just a plain typo.

Signed-off-by: Zhu Yanhai <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ Since this is an ABI and an old bug, we'll test this via a
  slow upstream route, to hopefully discover any app breakage. ]
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/fair.c

@@ -6101,7 +6101,7 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
 	 * idle runqueue:
 	 */
 	if (rq->cfs.load.weight)
-		rr_interval = NS_TO_JIFFIES(sched_slice(&rq->cfs, se));
+		rr_interval = NS_TO_JIFFIES(sched_slice(cfs_rq_of(se), se));
 
 	return rr_interval;
 }