ethernet: use likely() for common Ethernet encap

Mark code path's likely/unlikely based on most common usage.
  * Very few devices use dsa tags.
  * Most traffic is Ethernet (not 802.2)
  * No sane person uses trailer type or Novell encapsulation

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethernet/eth.c

@@ -179,12 +179,13 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 	 * variants has been configured on the receiving interface,
 	 * and if so, set skb->protocol without looking at the packet.
 	 */
-	if (netdev_uses_dsa_tags(dev))
+	if (unlikely(netdev_uses_dsa_tags(dev)))
 		return htons(ETH_P_DSA);
-	if (netdev_uses_trailer_tags(dev))
+
+	if (unlikely(netdev_uses_trailer_tags(dev)))
 		return htons(ETH_P_TRAILER);
 
-	if (ntohs(eth->h_proto) >= ETH_P_802_3_MIN)
+	if (likely(ntohs(eth->h_proto) >= ETH_P_802_3_MIN))
 		return eth->h_proto;
 
 	/*
@@ -193,7 +194,7 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 	 *      layer. We look for FFFF which isn't a used 802.2 SSAP/DSAP. This
 	 *      won't work for fault tolerant netware but does for the rest.
 	 */
-	if (skb->len >= 2 && *(unsigned short *)(skb->data) == 0xFFFF)
+	if (unlikely(skb->len >= 2 && *(unsigned short *)(skb->data) == 0xFFFF))
 		return htons(ETH_P_802_3);
 
 	/*