LinuxKPI: Implement linux/hashtable.h for FreeBSD.

This implementation uses the concurrency kit, CK, API directly which is
suitable for use with EPOCH(9) and RCU under FreeBSD.

No functional change intended.

The initial "linux/hash.h" code was obtained from DragonFlyBSD via
FreeBSD's drm-kmod in ports.

Differential Revision: https://reviews.freebsd.org/D35162
Reviewed by: bz@ and markj@
MFC after: 1 week
Sponsored by: NVIDIA Networking