netfilter: flowtable: incorrect pppoe tuple

pppoe traffic reaching ingress path does not match the flowtable entry
because the pppoe header is expected to be at the network header offset.
This bug causes a mismatch in the flow table lookup, so pppoe packets
enter the classical forwarding path.

Fixes: 72efd585 ("netfilter: flowtable: add pppoe support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_ip.c

@@ -157,7 +157,7 @@ static void nf_flow_tuple_encap(struct sk_buff *skb,
 		tuple->encap[i].proto = skb->protocol;
 		break;
 	case htons(ETH_P_PPP_SES):
-		phdr = (struct pppoe_hdr *)skb_mac_header(skb);
+		phdr = (struct pppoe_hdr *)skb_network_header(skb);
 		tuple->encap[i].id = ntohs(phdr->sid);
 		tuple->encap[i].proto = skb->protocol;
 		break;