LinuxKPI: add cpu.h for cpumask_*()
Add linux/cpu.h for cpumask_*() functions found in wireless drivers
and make sure cpu_online_mask is always initialised.
Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30421
(cherry picked from commit 5fce802722cd4435a748d89043615324db4efe5e)