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 --cc include/linux/sched.h
Simple merge

diff --cc kernel/acct.c
Simple merge

diff --cc kernel/fork.c
Simple merge

diff --cc kernel/tsacct.c
Simple merge