s390/cputime: provide archicture specific cputime_to_nsecs

The generic cputime_to_nsecs function first converts the cputime
to micro-seconds and then multiplies the result with 1000. This
looses some bits of accuracy, provide our own version of
cputime_to_nsecs that does not loose precision.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/cputime.h

@@ -8,6 +8,7 @@
 #define _S390_CPUTIME_H
 
 #include <linux/types.h>
+#include <asm/timex.h>
 
 #define CPUTIME_PER_USEC 4096ULL
 #define CPUTIME_PER_SEC (CPUTIME_PER_USEC * USEC_PER_SEC)
@@ -17,13 +18,18 @@
 #define cmpxchg_cputime(ptr, old, new) cmpxchg64(ptr, old, new)
 
 /*
- * Convert cputime to microseconds and back.
+ * Convert cputime to microseconds.
  */
 static inline u64 cputime_to_usecs(const u64 cputime)
 {
 	return cputime >> 12;
 }
 
+/*
+ * Convert cputime to nanoseconds.
+ */
+#define cputime_to_nsecs(cputime) tod_to_ns(cputime)
+
 u64 arch_cpu_idle_time(int cpu);
 
 #define arch_idle_time(cpu) arch_cpu_idle_time(cpu)