xen: use non-tracing preempt in xen_clocksource_read()

The tracing code used sched_clock() to get tracing timestamps, which
ends up calling xen_clocksource_read().  xen_clocksource_read() must
disable preemption, but if preemption tracing is enabled, this results
in infinite recursion.

I've only noticed this when boot-time tracing tests are enabled, but it
seems like a generic bug.  It looks like it would also affect
kvm_clocksource_read().

Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/xen/time.c

@@ -168,9 +168,10 @@ cycle_t xen_clocksource_read(void)
         struct pvclock_vcpu_time_info *src;
 	cycle_t ret;
 
-	src = &get_cpu_var(xen_vcpu)->time;
+	preempt_disable_notrace();
+	src = &__get_cpu_var(xen_vcpu)->time;
 	ret = pvclock_clocksource_read(src);
-	put_cpu_var(xen_vcpu);
+	preempt_enable_notrace();
 	return ret;
 }