Pull hotplug API fix from Ingo Molnar:
"Late breaking fix from the v4.9 cycle: fix a hotplug register/
unregister notifier API asymmetry bug that can cause kernel warnings
(and worse) with certain Kconfig combinations"
* 'smp-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
hotplug: Make register and unregister notifier API symmetric