netfilter: nft_tunnel: fix geneve_opt dump

When dumping a nft_tunnel with more than one geneve_opt configured the
netlink attribute hierarchy should be as follow:

 NFTA_TUNNEL_KEY_OPTS
 |
 |--NFTA_TUNNEL_KEY_OPTS_GENEVE
 |  |
 |  |--NFTA_TUNNEL_KEY_GENEVE_CLASS
 |  |--NFTA_TUNNEL_KEY_GENEVE_TYPE
 |  |--NFTA_TUNNEL_KEY_GENEVE_DATA
 |
 |--NFTA_TUNNEL_KEY_OPTS_GENEVE
 |  |
 |  |--NFTA_TUNNEL_KEY_GENEVE_CLASS
 |  |--NFTA_TUNNEL_KEY_GENEVE_TYPE
 |  |--NFTA_TUNNEL_KEY_GENEVE_DATA
 |
 |--NFTA_TUNNEL_KEY_OPTS_GENEVE
 ...

Otherwise, userspace tools won't be able to fetch the geneve options
configured correctly.

Fixes: 925d8446 ("netfilter: nft_tunnel: add support for geneve opts")
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>