ipv6: route: extend flow representation with tunnel key

Use flowi_tunnel in flowi6 similarly to what is done with IPv4.
This complements commit 1b7179d3 ("route: Extend flow representation
with tunnel key").

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>