c69e855178
ip_forward() would report back a zero MTU in ICMP needfrag messages because on a IPSEC SP lookup failure no MTU got computed. Fix this by changing the logic to compute a new MTU in any case if IPSEC didn't do it. Change MTU computation logic to use egress interface MTU if available or the next smaller MTU compared to the current packet size instead of falling back to a very small fixed MTU. Fix associated comment. PR: kern/91412 MFC after: 3 days