LinuxKPI: add dummy rhashtable implementation

Add a skeleton implementation of rhashtable in order to keep an
upcoming wireless driver compiling. We'll implement it as soon as
we get there.

Reviewed by: emaste (previous version before his suggested changes)
Differential Revision: https://reviews.freebsd.org/D40176

(cherry picked from commit a30a0d940daef2df51371cf142790109e2019fa0)