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: 925d844696d9 ("netfilter: nft_tunnel: add support for geneve opts")
Signed-off-by: Fernando Fernandez Mancera <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nft_tunnel.c

···
 		struct geneve_opt *opt;
 		int offset = 0;
 
-		inner = nla_nest_start_noflag(skb, NFTA_TUNNEL_KEY_OPTS_GENEVE);
-		if (!inner)
-			goto failure;
 		while (opts->len > offset) {
+			inner = nla_nest_start_noflag(skb, NFTA_TUNNEL_KEY_OPTS_GENEVE);
+			if (!inner)
+				goto failure;
 			opt = (struct geneve_opt *)(opts->u.data + offset);
 			if (nla_put_be16(skb, NFTA_TUNNEL_KEY_GENEVE_CLASS,
 					 opt->opt_class) ||
···
 				    opt->length * 4, opt->opt_data))
 				goto inner_failure;
 			offset += sizeof(*opt) + opt->length * 4;
+			nla_nest_end(skb, inner);
 		}
-		nla_nest_end(skb, inner);
 	}
 	nla_nest_end(skb, nest);
 	return 0;