LinuxKPI: add kvmemdup()

Add kvmemdup() as a variant of kmemdup(). While currently it could
just call kmemdup() we duplicate the code and use kvmalloc() in case
someone will change the implementation of kvmalloc/kvfree in slab.h.
This is used by an updated wireless driver.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D45181