ANDROID: Export kthread_set_per_cpu

kthread_create_on_cpu no longer marks the created thread as a per cpu
thread, so the affinity might get lost on suspend or other hotplug
events.

Export kthread_set_per_cpu so a module that needs a kthread to stay on a
specific cpu can accomplish that.

Bug: 274202992
Change-Id: Iaafc12f93f341f9e0586cb051b7f1c941f140866
Signed-off-by: Arve Hjønnevåg <arve@android.com>