[ospfd] Minor enhancements to recent self-host-routes suppression patch

[ospfd] Minor enhancements to recent self-host-routes suppression patch

	* ospf_spf.c: (ospf_spf_process_stubs) Track whether
	  parent router vertex is the root, so that the host-route
	  suppression logic need only be activated for such vertices.
	  Move the actual logic to ospf_intra_add_stub.
	* ospf_route.c: (ospf_intra_add_stub) Main test of link moved
	  here, notionally more appropriate.