sched: fix rt-bandwidth hotplug race

When we hot-unplug a cpu and rebuild the sched-domain, all cpus will be
detatched. Alex observed the case where a runqueue was stealing bandwidth
from an already disabled runqueue to satisfy its own needs.

Stop this by skipping over already disabled runqueues.
Reported-by: NAlex Nixon <alex.nixon@citrix.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: NAlex Nixon <alex.nixon@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched_rt.c

@@ -298,7 +298,7 @@ static void __disable_runtime(struct rq *rq)
 			struct rt_rq *iter = sched_rt_period_rt_rq(rt_b, i);
 			s64 diff;
 
-			if (iter == rt_rq)
+			if (iter == rt_rq || iter->rt_runtime == RUNTIME_INF)
 				continue;
 
 			spin_lock(&iter->rt_runtime_lock);