Remove reference to kthread_create_on_cpu

kthread_create_on_cpu doesn't exist so update a comment in kthread.c to reflect
this.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>