sched: Make task->start_time nanoseconds based

Simplify the timespec to nsec/usec conversions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 67678fa..10c6e82 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1367,7 +1367,7 @@
 	} vtime_snap_whence;
 #endif
 	unsigned long nvcsw, nivcsw; /* context switch counts */
-	struct timespec start_time; 		/* monotonic time */
+	u64 start_time;		/* monotonic time in nsec */
 	u64 real_start_time;	/* boot based time in nsec */
 /* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
 	unsigned long min_flt, maj_flt;