pfctl: Fill netmask AF-independently in print_host()

Instead of masking the host address in two different ways, just fill it
no matter the address family. In case of AF_INET, setting the extra 96 bit
does not hurt.

While here, stop resetting `af' for no reason and move up the variable
declaration.

OK benno sashan

Obtained from: OpenBSD, kn <kn@openbsd.org>, 53f568abff
Sponsored by: Rubicon Communications, LLC ("Netgate")