usbnet: Fix tx_packets stat for FLAG_MULTI_FRAME drivers

Currently the usbnet core does not update the tx_packets statistic for
drivers with FLAG_MULTI_PACKET and there is no hook in the TX
completion path where they could do this.

cdc_ncm and dependent drivers are bumping tx_packets stat on the
transmit path while asix and sr9800 aren't updating it at all.

Add a packet count in struct skb_data so these drivers can fill it
in, initialise it to 1 for other drivers, and add the packet count
to the tx_packets statistic on completion.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/usb/usbnet.h b/include/linux/usb/usbnet.h
index d9a4905..ff3fb2b 100644
--- a/include/linux/usb/usbnet.h
+++ b/include/linux/usb/usbnet.h
@@ -228,8 +228,20 @@
 	struct usbnet		*dev;
 	enum skb_state		state;
 	size_t			length;
+	unsigned long		packets;
 };
 
+/* Drivers that set FLAG_MULTI_PACKET must call this in their
+ * tx_fixup method before returning an skb.
+ */
+static inline void
+usbnet_set_skb_tx_stats(struct sk_buff *skb, unsigned long packets)
+{
+	struct skb_data *entry = (struct skb_data *) skb->cb;
+
+	entry->packets = packets;
+}
+
 extern int usbnet_open(struct net_device *net);
 extern int usbnet_stop(struct net_device *net);
 extern netdev_tx_t usbnet_start_xmit(struct sk_buff *skb,