netfilter: nft_payload: rebuild vlan header when needed

Skip rebuilding the vlan header when accessing destination and source
mac address.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_payload.c

@@ -171,7 +171,8 @@ void nft_payload_eval(const struct nft_expr *expr,
 		if (!skb_mac_header_was_set(skb))
 			goto err;
 
-		if (skb_vlan_tag_present(skb)) {
+		if (skb_vlan_tag_present(skb) &&
+		    priv->offset >= offsetof(struct ethhdr, h_proto)) {
 			if (!nft_payload_copy_vlan(dest, skb,
 						   priv->offset, priv->len))
 				goto err;