net: release skb->dst in sock_queue_rcv_skb()

When queuing a skb to sk->sk_receive_queue, we can release its dst,
not anymore needed.  Since current cpu did the dst_hold(), refcount is
probably still hot int this cpu caches.

This avoids readers to access the original dst to decrement its
refcount, possibly a long time after packet reception. This should
speedup UDP and RAW receive path.
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -289,7 +289,11 @@ int sock_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 
 	skb->dev = NULL;
 	skb_set_owner_r(skb, sk);
-
+	/*
+	 * release dst right now while its hot
+	 */
+	dst_release(skb->dst);
+	skb->dst = NULL;
 	/* Cache the SKB length before we tack it onto the receive
 	 * queue.  Once it is added it no longer belongs to us and
 	 * may be freed by other threads of control pulling packets