nm_ip_route_get_prefix() and plen are guint type, hence the following
is not correct:

    plen = nm_ip_route_get_prefix (route1);
    r = plen - nm_ip_route_get_prefix (route2);
    if (r)
         return r > 0 ? 1 : -1;

Use the macro, it gets subtle cases like this right.

Fixes: b32bb36c