Before commit 6698bf58, we would rely on
kernel to add the device-route for manual IPv6 routes. We broke that and now
kernel would still add the device-route, however nm_platform_ip_route_sync()
would delete it immediately after.
That is because previously nm_platform_ip_route_sync() would ignore routes
with rtm_protocol RTPRO_KERNEL. Now, it will sync and delete those too.

Fix that by adding the device-route like we do it for IPv4. This also
fixes an actual issue where the automatically added route always had
route-metric 256. Instead, we now use the metric from ipv6.route-metric
setting.

Fixes: 6698bf58