cpu: hotplug: Preserve per instance callback errors

In the Linux kernel, the following vulnerability has been resolved: cpu: hotplug: Preserve per instance callback errors cpuhp_invoke_callback() unwinds earlier callbacks for the same hotplug state when one instance fails. The rollback path currently reuses ret, so a successful rollback can hide the original error and make the failed transition look successful. Keep the rollback result separate from the original error.