sched: maintain per-rq runnable averages

Since runqueues do not have a corresponding sched_entity we instead embed a
sched_avg structure directly.

Signed-off-by: Ben Segall <bsegall@google.com>
Reviewed-by: Paul Turner <pjt@google.com>
modified for Mako kernel from kernel.org

Signed-off-by: faux123 <reioux@gmail.com>