[WireGuard] Source address fib invalidation on IPv6

[Jason A. Donenfeld]

Hi Hannes,

On Sun, Nov 13, 2016 at 1:51 AM, Hannes Frederic Sowa
<hannes at stressinduktion.org> wrote:
> You probably need some combination of ipv6_chk_addr and/or
> ipv6_check_addr_and_flags (where dev can also be NULL). Be careful if a
> IFA_HOST or IFA_LINK address switches from one interface to another.

I can confirm this trick works beautifully:
https://git.zx2c4.com/WireGuard/commit/?id=eb65810fc6350c50b42abedd1291b12337d3dc3d

I'll see if I can fold this into the routing function so that it
behaves the same as v4, unless David gets there first.

Regards,
Jason