net: Avoid modulus in skb_tx_hash() for forwarding case.

Based almost entirely upon a patch by Eric Dumazet.

The common case is to have num-tx-queues <= num_rx_queues
and even if num_tx_queues is larger it will not be significantly
larger.

Therefore, a subtraction loop is always going to be faster than
modulus.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1735,8 +1735,12 @@ u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
 {
 	u32 hash;
 
-	if (skb_rx_queue_recorded(skb))
-		return skb_get_rx_queue(skb) % dev->real_num_tx_queues;
+	if (skb_rx_queue_recorded(skb)) {
+		hash = skb_get_rx_queue(skb);
+		while (unlikely (hash >= dev->real_num_tx_queues))
+			hash -= dev->real_num_tx_queues;
+		return hash;
+	}
 
 	if (skb->sk && skb->sk->sk_hash)
 		hash = skb->sk->sk_hash;