The attempt to prevent overwriting an active state resulted in a
disaster which effectively disables all dynamically allocated hotplug
states.

Cleanup the mess.

Fixes: dc280d93 ("cpu/hotplug: Prevent overwriting of callbacks")
Reported-by: NMarkus Trippelsdorf <markus@trippelsdorf.de>
Reported-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>