clocksource: Make delta calculation a function

We want to move the TSC sanity check into core code to make NMI safe
accessors to clock monotonic[_raw] possible. For this we need to
sanity check the delta calculation. Create a helper function and
convert all sites to use it.

[ Build fix from jstultz ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h
index e3d28ad..05dfa6b 100644
--- a/kernel/time/timekeeping_internal.h
+++ b/kernel/time/timekeeping_internal.h
@@ -3,6 +3,7 @@
 /*
  * timekeeping debug functions
  */
+#include <linux/clocksource.h>
 #include <linux/time.h>
 
 #ifdef CONFIG_DEBUG_FS
@@ -11,4 +12,9 @@
 #define tk_debug_account_sleep_time(x)
 #endif
 
+static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
+{
+	return (now - last) & mask;
+}
+
 #endif /* _TIMEKEEPING_INTERNAL_H */