sched/fair: use rq->nr_running when balancing load

cfs load_balance only takes care of CFS tasks whereas CPUs can be used by
other scheduling class. Typically, a CFS task preempted by a RT or deadline
task will not get a chance to be pulled on another CPU because the
load_balance doesn't take into account tasks from other classes.
Add sum of nr_running in the statistics and use it to detect such
situation.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>