sched: Make task->real_start_time nanoseconds based

Simplify the only user of this data by removing the timespec
conversion.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

diff --cc fs/proc/array.c
Simple merge

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

diff --cc kernel/fork.c
Simple merge