netfilter: nft_tunnel: also dump ERSPAN_VERSION

This is not necessary, but it'll be easier to parse in userspace,
also given that other places like act_tunnel_key, cls_flower and
ip_tunnel_core are also doing so.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>