Fix vlan header offset in __skb_flow_dissect

[Fengtiantian]

We use the openvswitch 2.7.0 and find the issue when ovs use
the skb_get_hash() to get the hash of QinQ skb. Because the
__skb_flow_dissect() get the wrong vlan protocol headers.

Someone report bonding driver has the same issue use the
__skb_flow_dissect() to count hash in bond_xmit_hash:
https://lore.kernel.org/netdev/00a5d09f-a23e-661f-60c0-
75fba6227451@xxxxxxxxxx/T/.

Because in netif_receive_skb, the skb_network_header points
to vlan head, but in dev_hard_start_xmit, the skb_network_header
points to IP header. So use the skb_network_offset to get the
vlan head is not reliable.

Should we use the skb_mac_offset instead the skb_network_offset
to get the vlan head when proto is ETH_P_8021AD or ETH_P_8021Q?

Signed-off-by: Feng tiantian <fengtiantian@xxxxxxxxxx>
---
 net/core/flow_dissector.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 415b95f..9a77d5d 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -629,6 +629,13 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 			 skb->vlan_proto : skb->protocol;
 		nhoff = skb_network_offset(skb);
 		hlen = skb_headlen(skb);
+
+		if (proto == htons(ETH_P_8021AD) ||
+		    proto == htons(ETH_P_8021Q)) {
+			if (skb_mac_header_was_set(skb))
+				nhoff = skb_mac_offset(skb) + ETH_HLEN;
+		}
+
 #if IS_ENABLED(CONFIG_NET_DSA)
 		if (unlikely(skb->dev && netdev_uses_dsa(skb->dev))) {
 			const struct dsa_device_ops *ops;
-- 
1.8.3.1