sched: Avoid pulling all tasks from a CPU during load balance

When running load balance, the destination CPU checks the number
of running tasks on the busiest CPU without holding the busiest
CPUs runqueue lock. This opens the load balancer to a race whereby
a third CPU running load balance at the same time; having found the
same busiest group and queue, may have already pulled one of the
waiting tasks from the busiest CPU. Under scenarios where the source
CPU is running the idle task and only a single task remains waiting on
the busiest runqueue (nr_running = 1), the destination CPU will end
up pulling the only enqueued task from that CPU, leaving the destination
CPU with nothing left to run. Fix this race, by reconfirming nr_running
for the busiest CPU, after its runqueue lock has been obtained.

Change-Id: I42e132b15f96d9d5d7b32ef4de3fb92d2f837e63
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>