platform: skip metric-0 IPv6 routes in nm_platform_ip_route_sync()

@routes are the list of routes we want to configure. This contains
routes from DHCP and manual routes in the profile. It also contains
externally present routes, including the metric=0 routes in the local
table.

Trying to add an IPv6 route with metric zero adds instead a route with
metric 1024.

Usually, we wouldn't do that, because that route was present externally,
so it possibly is still present (in the platform cache) during sync and
we skip the addition. However, there is a race where the external route
might just disappear and we'd add a route with metric 1024.

Avoid that.

src/platform/nm-platform.c

@@ -4323,6 +4323,13 @@ nm_platform_ip_route_sync (NMPlatform *self,
 				continue;
 			}
 
+			if (   !IS_IPv4
+			    && NMP_OBJECT_CAST_IP6_ROUTE (conf_o)->metric == 0) {
+				/* User space cannot add routes with metric 0. However, kernel can, and we might track such
+				 * routes in @route as they are present external. Skip them silently. */
+				continue;
+			}
+
 			plat_entry = nm_platform_lookup_entry (self,
 			                                       NMP_CACHE_ID_TYPE_OBJECT_TYPE,
 			                                       conf_o);