pf: ensure we have the correct source/destination IP address in ICMP errors

When we route-to a packet that later turns out to not fit in the
outbound interface MTU we generate an ICMP error.
However, if we've already changed those (i.e. we've passed through a NAT
rule) we have to undo the transformation first.

Obtained from: pfSense
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32571