pf: fix quoted ip packet length for af-to
With address family translation, the ip length of the quoted ip
packet within the icmp error packet was wrong. Fix this by using
the pd2.tot_len of the inner packet and substract the old header's
length.
OK mikeb@ henning@
Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, 3d0247e0e8
Sponsored by: Rubicon Communications, LLC ("Netgate")