core: use NM_CMP_*() macro in route_compare()

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
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