pf: pass pre-NAT addresses to dummynet

When we're NAT-ing give dummynet (via its struct ip_fw_args) the pre-NAT source
and destination addresses.
That's what we used to do, but that got unintentionally changed during the nat64
work.

The pre-NAT addresses make somewhat more sense, in that it enables limiting
based on specific LAN clients.

See also: https://redmine.pfsense.org/issues/15770
Sponsored by: Rubicon Communications, LLC ("Netgate")