net: flow_dissector: fix RPS on DSA masters

After the blamed patch, __skb_flow_dissect() on the DSA master stopped
adjusting for the length of the DSA headers. This is because it was told
to adjust only if the needed_headroom is zero, aka if there is no DSA
header. Of course, the adjustment should be done only if there _is_ a
DSA header.

Modify the comment too so it is clearer.

Fixes: 4e500251 ("net: dsa: generalize overhead for taggers that use both headers and trailers")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -943,8 +943,8 @@ bool __skb_flow_dissect(const struct net *net,
 			int offset = 0;
 
 			ops = skb->dev->dsa_ptr->tag_ops;
-			/* Tail taggers don't break flow dissection */
-			if (!ops->needed_headroom) {
+			/* Only DSA header taggers break flow dissection */
+			if (ops->needed_headroom) {
 				if (ops->flow_dissect)
 					ops->flow_dissect(skb, &proto, &offset);
 				else