sched: reduce overestimating rq->avg_idle, When updating avg_idle, if the delta exceeds some max value, then avg_idle
gets set to the max, regardless of what the previous avg was. This can cause
avg_idle to often be overestimated.

This patch modifies the way we update avg_idle by always updating it with the
function call to update_avg() first. Then, if avg_idle exceeds the max, we set
it to the max.

Change-Id: I05d66b4dce4c6fb6a709302de9ab6c8ec4ed3aac
Signed-off-by: zwliew <zhaoweiliew@gmail.com>