pf: fix pf_ioctl_add_addr() validation

Ensure we can only pass AF_UNSPEC, AF_INET or AF_INET6 (and only when supported
in our kernel config).

Reported-by: syzbot+8a9ee157bfed9e6b9efc@syzkaller.appspotmail.com
Sponsored by: Rubicon Communications, LLC ("Netgate")