netfilter: nft_tunnel: also dump OPTS_ERSPAN/VXLAN

This patch is to add the nest attr OPTS_ERSPAN/VXLAN when dumping
KEY_OPTS, and it would be helpful when parsing in userpace. Also,
this is needed for supporting multiple geneve opts in the future
patches.

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>