cpuidle: Fix idle time tracking

Ville reported that on his Core2, which has TSC stop in idle, we would
always report very short idle durations. He tracked this down to
commit:

  e93e59ce ("cpuidle: Replace ktime_get() with local_clock()")

which replaces ktime_get() with local_clock().

Add a sched_clock_idle_wakeup_event() call, which will re-sync the
clock with ktime_get_ns() when TSC is unstable and no-op otherwise.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J . Wysocki <rafael.j.wysocki@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Fixes: e93e59ce ("cpuidle: Replace ktime_get() with local_clock()")
Signed-off-by: Ingo Molnar <mingo@kernel.org>