s390/ftrace: fix endless recursion in function_graph tracer

The following sequence triggers a kernel stack overflow on s390x:

mount -t tracefs tracefs /sys/kernel/tracing
cd /sys/kernel/tracing
echo function_graph > current_tracer
[crash]

This is because preempt_count_{add,sub} are in the list of traced
functions, which can be demonstrated by:

echo preempt_count_add >set_ftrace_filter
echo function_graph > current_tracer
[crash]

The stack overflow happens because get_tod_clock_monotonic() gets called
by ftrace but itself calls preempt_{disable,enable}(), which leads to a
endless recursion. Fix this by using preempt_{disable,enable}_notrace().

Fixes: 01162068 ("s390/time: ensure get_clock_monotonic() returns monotonic values")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/timex.h

@@ -194,9 +194,9 @@ static inline unsigned long long get_tod_clock_monotonic(void)
 {
 	unsigned long long tod;
 
-	preempt_disable();
+	preempt_disable_notrace();
 	tod = get_tod_clock() - *(unsigned long long *) &tod_clock_base[1];
-	preempt_enable();
+	preempt_enable_notrace();
 	return tod;
 }