net: fix double-tcpdump problem with NETDEV_TX_BUSY.

Herbert shares a distain for drivers returning TX_BUSY because
network taps see packets twice when it's used.  Unfortunately, it's
ubiquitous.

This patch marks packets by (ab)using the "peeked" bit in the skb.
This bit is currently used for packets queued in a socket; we reset it
in dev_queue_xmit and set it when we hand the packet to
dev_queue_xmit_nit.

We also reset it on incoming packets: this is safe, but it might be
sufficient to reset it only in the loopback driver?
---
 net/core/dev.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1678,8 +1678,10 @@ int dev_hard_start_xmit(struct sk_buff *
 	int rc;
 
 	if (likely(!skb->next)) {
-		if (!list_empty(&ptype_all))
+		if (!list_empty(&ptype_all) && !skb->peeked) {
 			dev_queue_xmit_nit(skb, dev);
+			skb->peeked = true;
+		}
 
 		if (netif_needs_gso(dev, skb)) {
 			if (unlikely(dev_gso_segment(skb)))
@@ -1796,6 +1798,8 @@ int dev_queue_xmit(struct sk_buff *skb)
 	struct Qdisc *q;
 	int rc = -ENOMEM;
 
+	skb->peeked = false;
+
 	/* GSO will handle the following emulations directly. */
 	if (netif_needs_gso(dev, skb))
 		goto gso;
@@ -1942,6 +1946,8 @@ int netif_rx(struct sk_buff *skb)
 	if (!skb->tstamp.tv64)
 		net_timestamp(skb);
 
+	skb->peeked = false;
+
 	/*
 	 * The code is rearranged so that the path is the most
 	 * short when CPU is congested, but is still operating.