[NETFILTER]: bridge: fix double POST_ROUTING invocation

The bridge code incorrectly causes two POST_ROUTING hook invocations
for DNATed packets that end up on the same bridge device. This
happens because packets with a changed destination address are passed
to dst_output() to make them go through the neighbour output function
again to build a new destination MAC address, before they will continue
through the IP hooks simulated by bridge netfilter.

The resulting hook order is:
PREROUTING (bridge netfilter)
POSTROUTING (dst_output -> ip_output)
FORWARD (bridge netfilter)
POSTROUTING (bridge netfilter)

The deferred hooks used to abort the first POST_ROUTING invocation,
but since the only thing bridge netfilter actually really wants is
a new MAC address, we can avoid going through the IP stack completely
by simply calling the neighbour output function directly.

Tested, reported and lots of data provided by: Damien Thebault <[e-mail]>

Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_netfilter.c

···
  * Let us first consider the case that ip_route_input() succeeds:
  *
  * If skb->dst->dev equals the logical bridge device the packet
- * came in on, we can consider this bridging. We then call
- * skb->dst->output() which will make the packet enter br_nf_local_out()
+ * came in on, we can consider this bridging. The packet is passed
+ * through the neighbour output function to build a new destination
+ * MAC address, which will make the packet enter br_nf_local_out()
  * not much later. In that function it is assured that the iptables
  * FORWARD chain is traversed for the packet.
  *
···
 	skb->nf_bridge->mask ^= BRNF_NF_BRIDGE_PREROUTING;
 
 	skb->dev = bridge_parent(skb->dev);
-	if (!skb->dev)
-		kfree_skb(skb);
-	else {
+	if (skb->dev) {
+		struct dst_entry *dst = skb->dst;
+
 		nf_bridge_pull_encap_header(skb);
-		skb->dst->output(skb);
+
+		if (dst->hh)
+			return neigh_hh_output(dst->hh, skb);
+		else if (dst->neighbour)
+			return dst->neighbour->output(skb);
 	}
+	kfree_skb(skb);
 	return 0;
 }