network: update state file after dhcp6 events

E.g. DNS servers may be received from DHCPv6 server. If the link is
already in configured state, the DNS servers are not written in the
state file.

src/network/networkd-dhcp6.c

@@ -497,6 +497,7 @@ static void dhcp6_handler(sd_dhcp6_client *client, int event, void *userdata) {
                 (void) dhcp6_lease_pd_prefix_lost(client, link);
                 (void) dhcp6_prefix_remove_all(link->manager, link);
 
+                link_dirty(link);
                 link->dhcp6_configured = false;
                 break;
 
@@ -519,6 +520,7 @@ static void dhcp6_handler(sd_dhcp6_client *client, int event, void *userdata) {
                         return;
                 }
 
+                link_dirty(link);
                 link->dhcp6_configured = true;
                 break;