timers: split process wide cpu clocks/timers, remove spurious warning

Mike Galbraith reported that the new warning in thread_group_cputimer()
triggers en masse with Amarok running.

Oleg Nesterov observed:

  Can't fastpath_timer_check()->thread_group_cputimer() have the
  false warning too? Suppose we had the timer, then posix_cpu_timer_del()
  removes this timer, but task_cputime_zero(&sig->cputime_expires) still
  not true.

Remove the spurious debug warning.
Reported-by: Mike Galbraith <efault@gmx.de>
Explained-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -2208,8 +2208,6 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
 	struct thread_group_cputimer *cputimer = &tsk->signal->cputimer;
 	unsigned long flags;
 
-	WARN_ON(!cputimer->running);
-
 	spin_lock_irqsave(&cputimer->lock, flags);
 	*times = cputimer->cputime;
 	spin_unlock_irqrestore(&cputimer->lock, flags);