sched/rt: Keep period timer ticking when rt throttling is active

When a runqueue is throttled we cannot disable the period timer
because that timer is the only way to undo the throttling.

We got stale throttling entries when a rq was throttled and then the
global sysctl was disabled, which stopped the timer.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
[ Added changelog ]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/n/tip-nuj34q52p6ro7szapuz84i0v@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>