net: dsa: Implement flow_dissect callback for tag_dsa.

RPS not work for DSA devices since the 'skb_get_hash'
will always get the invalid hash for dsa tagged packets.

"[PATCH] tag_mtk: add flow_dissect callback to the ops struct"
introduced the flow_dissect callback to get the right hash for
MTK tagged packet. Tag_dsa and tag_edsa  also need to implement
the callback.
Signed-off-by: Rundong Ge <rdong.ge@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_dsa.c

@@ -146,8 +146,17 @@ static struct sk_buff *dsa_rcv(struct sk_buff *skb, struct net_device *dev,
 	return skb;
 }
 
+static int dsa_tag_flow_dissect(const struct sk_buff *skb, __be16 *proto,
+				int *offset)
+{
+	*offset = 4;
+	*proto = ((__be16 *)skb->data)[1];
+	return 0;
+}
+
 const struct dsa_device_ops dsa_netdev_ops = {
 	.xmit	= dsa_xmit,
 	.rcv	= dsa_rcv,
+	.flow_dissect   = dsa_tag_flow_dissect,
 	.overhead = DSA_HLEN,
 };

net/dsa/tag_edsa.c

@@ -165,8 +165,17 @@ static struct sk_buff *edsa_rcv(struct sk_buff *skb, struct net_device *dev,
 	return skb;
 }
 
+static int edsa_tag_flow_dissect(const struct sk_buff *skb, __be16 *proto,
+				 int *offset)
+{
+	*offset = 8;
+	*proto = ((__be16 *)skb->data)[3];
+	return 0;
+}
+
 const struct dsa_device_ops edsa_netdev_ops = {
 	.xmit	= edsa_xmit,
 	.rcv	= edsa_rcv,
+	.flow_dissect   = edsa_tag_flow_dissect,
 	.overhead = EDSA_HLEN,
 };