Fix IPv6 ECMP handling with 4.11+ kernel

[Lennert Buytenhek]

On Fri, Sep 01, 2017 at 11:36:01AM +0200, Ondrej Zajicek wrote:

> > Starting from kernel 4.11 (commit beb1afac518d), IPv6 are now notified
> > using RTA_MULTIPATH, like IPv4 routes. Those routes are not handled
> > correctly by BIRD. We handle them correctly. This also enable to parse
> > alien routes correctly. Route modifications is still done in the old
> > way as for insertion/deletion, this is not helpful to optimize in those
> > cases and for replace, IPv4 case is not optimized either. It should be
> > possible to detect appropriate support for RTA_MULTIPATH when receiving
> > an IPv6 route with this attribute, but I don't see how it would be
> > helpful, so I didn't do it (simpler code this way).
> 
> Hi
> 
> Thanks for the patch, it looks OK. I will try it and merge it.
> 
> We plan to simplify the multipath processing code in 2.x branch so that
> it will depend on the new kernel behavior (and therefore will require
> Linux 4.0.11 for IPv6 multipath), but we keep the current code in 1.6.x.

We have lots of kit (e.g. https://www.arista.com/en/products/7280r-series)
that comes with:

	Linux labtest 3.4.43.Ar-3964936.4173F #1 SMP PREEMPT Mon Dec 19 14:31:21 PST 2016 x86_64 x86_64 x86_64 GNU/Linux

And which is impossible for us to upgrade to newer kernel versions.
If bird 2.0 will require Linux >= 4.11 for IPv6 multipath, then
upgrading to bird 2.0 would not be an option for us.