Remove 'struct route_int6' argument from in6_selectsrc() and
in6_selectif().
The main task of in6_selectsrc() is to return IPv6 SAS (along with
output interface used for scope checks). No data-path code uses route argument for caching. The only users are icmp6 (reflect code), ND6 ns/na generation code. All this fucntions are control-plane, so there is no reason to try to 'optimize' something by passing cached route into to ip6_output(). Given that, simplify code by eliminating in6_selectsrc() 'struct route_in6' argument. Since in6_selectif() is used only by in6_selectsrc(), eliminate its 'struct route_in6' argument, too. While here, reshape rte-related code inside in6_selectif() to free lookup result immediately after saving all the needed fields.