in_pcb: use jenkins hash over the entire IPv6 (or IPv4) address

The intent is to provide more entropy than can be provided
by just the 32-bits of the IPv6 address which overlaps with
6to4 tunnels. This is needed to mitigate potential algorithmic
complexity attacks from attackers who can control large
numbers of IPv6 addresses.

Together with: gallatin
Reviewed by: dwmalone, rscheff
Differential revision: https://reviews.freebsd.org/D33254