The IPv6 Router Advertisement (RA) MTU is now derived from
the minimum of the interface MTU and the MTUs of all IPv6 routes
on the upstream interface, before being clamped to the 1280-1400 range.
This ensures the advertised MTU respects any smaller
MTUs specified by individual routes.
Bug: 432676104
Test: Treehugger
Flag: EXEMPT mainline
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: If371825bd8f773ae23d1122211caa42ebf5c1f2a