sched: don't account latency of newly forking task

sched_task_util_* trace events account latency of select_task_rq_fair()
by using p->ravg.mark_start which isn't set for newly forking tasks.

Don't account latency when mark_start isn't set.

Change-Id: Icdd5acb1969cbe71a57f580f9d9bb8a46e329832
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>