hwmon: (coretemp) Simplify platform device handling

In the Linux kernel, the following vulnerability has been resolved:

hwmon: (coretemp) Simplify platform device handling

Coretemp's platform driver is unconventional. All the real work is done globally by the initcall and CPU hotplug notifiers, while the "driver" effectively just wraps an allocation and the registration of the hwmon interface in a long-winded round-trip through the driver core. The whole logic of dynamically creating and destroying platform devices to bring the interfaces up and down is error prone, since it assumes platform_device_add() will synchronously bind the driver and set drvdata before it returns, thus results in a NULL dereference if drivers_autoprobe is turned off for the platform bus. Furthermore, the unusual approach of doing that from within a CPU hotplug notifier, already commented in the code that it deadlocks suspend, also causes lockdep issues for other drivers or subsystems which may want to legitimately register a CPU hotplug notifier from a platform bus notifier.

All of these issues can be solved by ripping this unusual behaviour out completely, simply tying the platform devices to the lifetime of the module itself, and directly managing the hwmon interfaces from the hotplug notifiers. There is a slight user-visible change in that /sys/bus/platform/drivers/coretemp will no longer appear, and /sys/devices/platform/coretemp.n will remain present if package n is hotplugged off, but hwmon users should really only be looking for the presence of the hwmon interfaces, whose behaviour remains unchanged.