Fix IPv6 ECMP handling with 4.11+ kernel

[Vincent Bernat]

 ❦  1 septembre 2017 13:12 +0200, Ondrej Zajicek <santiago at crfreenet.org> :

>> > Also, alien routes are correctly parsed when next-hops are correctly
>> > ordered (I didn't check if this restriction is also present for IPv4 or
>> > if Linux is always sending IPv4 multipath routes with next-hops
>> > correctly ordered)
>> 
>> It's the same for IPv4. Dunno if it should be considered as a bug?
>
> Not sure what is a problem here. Do you mean that alien routes are not
> correctly parsed when next-hops are not ordered? How does the problem
> manifests?

Yes.

2017-09-01T13:19:20.741524+02:00 V1-1 bird6: Ignoring unsorted multipath route 2001:db8:a5::/64 received via kernel1

The route:

2001:db8:a5::/64 metric 1024
        nexthop via 2001:db8:ff::3  dev vti4 weight 1
        nexthop via 2001:db8:ff::1  dev vti3 weight 1

Same for IPv4.
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