time: prevent the loop in timespec_add_ns() from being optimised away

Since some architectures don't support __udivdi3().

--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -170,6 +170,10 @@ static inline void timespec_add_ns(struc
 {
 	ns += a->tv_nsec;
 	while(unlikely(ns >= NSEC_PER_SEC)) {
+		/* The following asm() prevents the compiler from
+		 * optimising this loop into a modulo operation.  */
+		asm("" : "+r"(ns));
+
 		ns -= NSEC_PER_SEC;
 		a->tv_sec++;
 	}