gre6: fix rtnl dump messages

Spotted after a code review.
Introduced by c12b395a (gre: Support GRE over
IPv6).

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>