network: dhcp6: explicitly specify metric for unreachable route

For static IPv6 routes without metric is specified, then we use 1024.
But such an adjustment is not performed to dynamic routes. So, let's
specify the metric explicitly.
Otherwise, configured routes will be handled as foreign.

src/network/networkd-dhcp6.c

@@ -777,6 +777,7 @@ static int dhcp6_request_unreachable_route(Link *link, const struct in6_addr *ad
         route->table = link_get_dhcp6_route_table(link);
         route->type = RTN_UNREACHABLE;
         route->protocol = RTPROT_DHCP;
+        route->priority = DHCP_ROUTE_METRIC;
         route->lifetime = usec_add(timestamp_usec, lifetime_sec * USEC_PER_SEC);
 
         if (route_get(link->manager, NULL, route, &existing) < 0)