network/route-metric: manage uint32_t RTAX_XYZ attributes in the same way

Except for RTAX_CC_ALGO, all RTAX_XYZ attributes take uint32_t and
the kernel's default value is zero. So, let's unify handling of the
attributes.

This should not change any effective behavior. Just refactoring.

src/network/networkd-route.c

@@ -35,7 +35,6 @@ int route_new(Route **ret) {
                 .type = RTN_UNICAST,
                 .table = RT_TABLE_MAIN,
                 .lifetime_usec = USEC_INFINITY,
-                .metric = ROUTE_METRIC_NULL,
                 .gateway_onlink = -1,
         };