breakage in policy routing on 5.3 and 5.3.1

[Jason A. Donenfeld]

Hi folks,

FYI, upstream Linux broke aspects policy routing in 5.3. This isn't
specific to WireGuard, but rather the policy routing engine in
general, but users of the wg-quick(8) bash script for configuring
networks might run into this if they're routing a default route and
have IPv6, because the bash script makes use of policy routing.

I sent two patches upstream to fix bugs introduced with 5.3:
https://lore.kernel.org/netdev/20190923144612.29668-1-Jason@zx2c4.com/
https://lore.kernel.org/netdev/20190924073615.31704-1-Jason@zx2c4.com/

The latter one appears to mitigate the problem, as confirmed by Arch
Linux users: https://bugs.archlinux.org/task/63870

I'll update this thread when we know more from the netdev mailing list.

Thanks,
Jason