sched: Consider PF_WAKE_UP_IDLE in select_best_cpu()

sysctl_sched_prefer_idle controls selection of idle cpus for waking
tasks. In some cases, waking to idle cpus help performance while in
other cases it hurts (as tasks incur latency associated with C-state
wakeup). Its ideal if scheduler can adapt prefer_idle behavior based
on the task that is waking up, but that's hard for scheduler to
figure by itself. PF_WAKE_UP_IDLE hint can be provided by external
module/driver in such case to guide scheduler in preferring an idle
cpu for select tasks irrespective of sysctl_sched_prefer_idle flag.

This patch enhances select_best_cpu() to consider PF_WAKE_UP_IDLE
hint. Wakeup posted from any task that has PF_WAKE_UP_IDLE set is a
hint for scheduler to prefer idle cpu for waking tasks. Similarly
scheduler will attempt to place any task with PF_WAKE_UP_IDLE set on
idle cpu when they wakeup.

Change-Id: Ia8bf334d98fd9fd2ff9eda875430497d55d64ce6
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>