pf: extra route lookup in pf_route(6)()

In the NAT64 case we use pf_route(6)() to emit the translated packet. This
requires a new route lookup, so perform this, but only in the NAT64/NAT46 case.
Update the destination sockaddr to send packets to the gateway if appropriate.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47788