[PATCH] NOHZ: Fix RCU handling

When a CPU is needed for RCU the tick has to continue even when it was
stopped before.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>