trace_clock: fix preemption bug

Using the function_graph tracer in recent kernels generates a spew of
preemption BUGs. Fix this by not requiring trace_clock_local() users
to disable preemption themselves.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/trace_clock.c

@@ -27,12 +27,19 @@
  */
 u64 notrace trace_clock_local(void)
 {
+	unsigned long flags;
+	u64 clock;
+
 	/*
 	 * sched_clock() is an architecture implemented, fast, scalable,
 	 * lockless clock. It is not guaranteed to be coherent across
 	 * CPUs, nor across CPU idle events.
 	 */
-	return sched_clock();
+	raw_local_irq_save(flags);
+	clock = sched_clock();
+	raw_local_irq_restore(flags);
+
+	return clock;
 }
 
 /*