Remove 'struct route_int6' argument from in6_selectsrc() and

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.