vpn: clear host part of IPv6 routes received from VPN plugin

Kernel would reject adding a route with a destination host part not
all zero. NetworkManager generally coerces such routes and there
are assertions in place to ensure that.

We forgot to ensure that for certain IPv6 routes from VPN plugins.
This can cause an assertion failure and wrong behavior.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/425

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/482

src/vpn/nm-vpn-connection.c

@@ -1790,6 +1790,8 @@ nm_vpn_connection_ip6_config_get (NMVpnConnection *self, GVariant *dict)
 			route.metric = route_metric;
 			route.rt_source = NM_IP_CONFIG_SOURCE_VPN;
 
+			nm_utils_ip6_address_clear_host_address (&route.network, &route.network, route.plen);
+
 			/* Ignore host routes to the VPN gateway since NM adds one itself.
 			 * Since NM knows more about the routing situation than the VPN
 			 * server, we want to use the NM created route instead of whatever