af_mpls: fix undefined reference to ip6_route_output

Undefined reference to ip6_route_output and ip_route_output
was reported with CONFIG_INET=n and CONFIG_IPV6=n.

This patch uses ipv6_stub_impl.ipv6_dst_lookup instead of
ip6_route_output. And wraps affected code under
IS_ENABLED(CONFIG_INET) and IS_ENABLED(CONFIG_IPV6).

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>