sched: monitor clock underflows in /proc/sched_debug

We monitor clock overflows, let's also monitor clock underflows.

Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index 80fbbfc..9e5de09 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -179,6 +179,7 @@
 	PN(prev_clock_raw);
 	P(clock_warps);
 	P(clock_overflows);
+	P(clock_underflows);
 	P(clock_deep_idle_events);
 	PN(clock_max_delta);
 	P(cpu_load[0]);