hotplug: simplify put_cpu_down() logic

This should make the code much more readable and probably
saves a few cpu cycles as well. In a best case scenario we'll
call get_cpu_load() only once and don't have to bother about
which cpu actually should be offlined.
In a worst case we should schedule twice and find the lowest
core. This should keep the latency very low during a hotunplug
operation.

Also fixing a small bug here where cpu_load_stats would have
the wrong cpu set to 0.