Update the MTU in affected routes when IPv6 RA changes the MTU

ip6_calcmtu() only looks at the interface MTU if neither the TCP hostcache
nor the route provides an MTU. Update the routes so they do not provide
stale MTUs.

This fixes UNH IPv6 conformance test cases v6LC_4_1_08 and v6LC_4_1_09,
which use a RA to reduce the link MTU from 1500 to 1280.

Reported and tested by: Farrell Woods <Farrell_Woods@Dell.com>
Reviewed by: dab, melifaro
Discussed with: ae
MFC after: 1 week
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D14257