sched: select task's prev_cpu as the best CPU when it was chosen recently

Select given task's prev_cpu when the task slept for short period to
reduce latency of task placement and migrations.  A new tunable
/proc/sys/kernel/sched_select_prev_cpu_us introduced to determine whether
tasks are eligible to go through fast path.

CRs-fixed: 947467
Change-Id: Ia507665b91f4e9f0e6ee1448d8df8994ead9739a
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>