netfilter: nft_tunnel: restrict it to netdev family

Only allow to use this expression from NFPROTO_NETDEV family.

Fixes: af308b94 ("netfilter: nf_tables: add tunnel support")
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_tunnel.c

@@ -161,6 +161,7 @@ static const struct nft_expr_ops nft_tunnel_get_ops = {
 
 static struct nft_expr_type nft_tunnel_type __read_mostly = {
 	.name		= "tunnel",
+	.family		= NFPROTO_NETDEV,
 	.ops		= &nft_tunnel_get_ops,
 	.policy		= nft_tunnel_policy,
 	.maxattr	= NFTA_TUNNEL_MAX,