powercap/intel_rapl: Track active CPUs internally

The ability of the CPU hotplug code to stop online/offline at each step
makes it necessary to track the activated CPUs in a package directly,
because outerwise a CPU offline callback can find CPUs which have already
executed the offline callback, but are not yet marked offline in the
topology mask. That could make such a CPU the package leader and in case
that CPU goes fully offline leave the package lead orphaned.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>