From: Roman Gushchin <redacted>
Date: Thu, 26 Mar 2015 14:49:54 +0300

This patch introduces new ipv6 sysctl: ra_default_route_mtu.
If it's set (> 0), it defines per-route MTU for any new default route
received by RA.

This sysctl will help in the following configuration: we want to use
jumbo-frames for internal networks and default ethernet frames for
default route. Per-route MTU can only lower per-link MTU, so link MTU
should be set to ~9000 (statically or via RA).

Due to dynamic nature of RA, setting MTU for default route will require
userspace agent, that will monitor changes of default route
and (re)configure it. Not simple. The suggested sysctl solves this
problem.

Signed-off-by: Roman Gushchin <redacted>

This does not apply cleanly to net-next, please respin.