sched/rt: Fix lockdep annotation within find_lock_lowest_rq()

Roland Dreier reported spurious, hard to trigger lockdep warnings
within the scheduler - without any real lockup.

This bit gives us the right clue:

> [89945.640512]  [<ffffffff8103fa1a>] double_lock_balance+0x5a/0x90
> [89945.640568]  [<ffffffff8104c546>] push_rt_task+0xc6/0x290

if you look at that code you'll find the double_lock_balance() in
question is the one in find_lock_lowest_rq() [yay for inlining].

Now find_lock_lowest_rq() has a bug.. it fails to use
double_unlock_balance() in one exit path, if this results in a retry in
push_rt_task() we'll call double_lock_balance() again, at which point
we'll run into said lockdep confusion.
Reported-by: Roland Dreier <roland@kernel.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1337282386.4281.77.camel@twinsSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/rt.c

@@ -1562,7 +1562,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 				     task_running(rq, task) ||
 				     !task->on_rq)) {
 
-				raw_spin_unlock(&lowest_rq->lock);
+				double_unlock_balance(rq, lowest_rq);
 				lowest_rq = NULL;
 				break;
 			}