usbnet: fix oops in usbnet_start_xmit

This patch fixes the bug added in commit v3.1-rc7-1055-gf9b491e
SKB can be NULL at this point, at least for cdc-ncm.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -1057,7 +1057,8 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 	unsigned long		flags;
 	int retval;
 
-	skb_tx_timestamp(skb);
+	if (skb)
+		skb_tx_timestamp(skb);
 
 	// some devices want funky USB-level framing, for
 	// win32 driver (usually) and/or hardware quirks