pf: fix IPv6 route lookup for nat64

The route lookup in pf_route6() (for the nat64 case) had the src/dst for bcopy()
swapped around, so any nat64 setup that passed via a gateway on the IPv6 side
was broken.

Fix this, add 'const' to make this error actually fail to compile and add a
test case.

Sponsored by: Rubicon Communications, LLC ("Netgate")