sched/rt: Enqueue just unthrottled rt_rq back on the stack in __disable_runtime()

Make rt_rq available for pick_next_task(). Otherwise, their tasks
stay prisoned long time till dead cpu becomes alive again.
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
CC: Konstantin Khorenko <khorenko@parallels.com>
CC: Ben Segall <bsegall@google.com>
CC: Paul Turner <pjt@google.com>
CC: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1403684388.3462.43.camel@tkhaiSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/rt.c

@@ -740,6 +740,9 @@ static void __disable_runtime(struct rq *rq)
 		rt_rq->rt_throttled = 0;
 		raw_spin_unlock(&rt_rq->rt_runtime_lock);
 		raw_spin_unlock(&rt_b->rt_runtime_lock);
+
+		/* Make rt_rq available for pick_next_task() */
+		sched_rt_rq_enqueue(rt_rq);
 	}
 }