netfilter: nft_tunnel: Add dst_cache support

The metadata_dst does not initialize the dst_cache field, this causes
problems to ip_md_tunnel_xmit() since it cannot use this cache, hence,
Triggering a route lookup for every packet.
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_tunnel.c

@@ -406,6 +406,13 @@ static int nft_tunnel_obj_init(const struct nft_ctx *ctx,
 		return -ENOMEM;
 
 	memcpy(&md->u.tun_info, &info, sizeof(info));
+#ifdef CONFIG_DST_CACHE
+	err = dst_cache_init(&md->u.tun_info.dst_cache, GFP_KERNEL);
+	if (err < 0) {
+		metadata_dst_free(md);
+		return err;
+	}
+#endif
 	ip_tunnel_info_opts_set(&md->u.tun_info, &priv->opts.u, priv->opts.len,
 				priv->opts.flags);
 	priv->md = md;