ipv6_stub: add ipv6_route_input stub/proxy.

Proxy ip6_route_input via ipv6_stub, for later use by lwt bpf ip encap
(see the next patch in the patchset).

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>