network: dhcp6pd: set default metric 256 for delegated prefix

When Assign= in [DHCPv6PrefixDelegation] is enabled, then the kernel
will create the prefix route for the assigned address with metric 256.
When Assign= is disabled, then the kernel will create the route with
metric 1024.

For the default value, we should choose a smaller value (higher priority)
than 1024, as the unreachable routes for delegated prefix will be
configured with 1024.

man/systemd.network.xml

@@ -2162,7 +2162,8 @@ Table=1234</programlisting></para>
         <term><varname>RouteMetric=</varname></term>
         <listitem>
           <para>The metric of the route to the delegated prefix subnet. Takes an unsigned integer in
-          the range 0…4294967295. When unset or set to 0, the kernel's default value is used.</para>
+          the range 0…4294967295. When set to 0, the kernel's default value is used. Defaults to 256.
+          </para>
         </listitem>
       </varlistentry>
     </variablelist>

src/network/networkd-dhcp-common.h

@@ -10,6 +10,7 @@
 #include "time-util.h"
 
 #define DHCP_ROUTE_METRIC 1024
+#define DHCP6PD_ROUTE_METRIC 256
 
 typedef struct Link Link;
 typedef struct Manager Manager;

src/network/networkd-network.c

@@ -409,7 +409,8 @@ int network_load_one(Manager *manager, OrderedHashmap **networks, const char *fi
                 .dhcp6_pd_announce = true,
                 .dhcp6_pd_assign = true,
                 .dhcp6_pd_manage_temporary_address = true,
-                .dhcp6_pd_subnet_id = -1,
+                .dhcp6_pd_subnet_id = UINT64_MAX,
+                .dhcp6_pd_route_metric = DHCP6PD_ROUTE_METRIC,
 
                 .dhcp_server_bind_to_interface = true,
                 .dhcp_server_emit[SD_DHCP_LEASE_DNS].emit = true,