sched: Use cached value of span instead of calling sched_domain_span()

In the beginning of build_sched_groups() we called sched_domain_span() and cached its return value in span. Few statements later we are calling it again to get the same pointer.

Lets use the cached value instead as it hasn't changed in between.
sched: don't call get_group() for covered cpus

In build_sched_groups() we don't need to call get_group() for cpus which are already covered in previous iterations. So, call get_group() after checking if cpu is covered or not.