usbnet: ratelimit kevent may have been dropped warnings

when something goes wrong, a flood of these messages can be
generated by usbnet (thousands per second).  This doesn't
generally *help* the condition so this patch ratelimits the
rate of their generation.

There's an underlying problem in usbnet's kevent deferral
mechanism which needs fixing, specifically that events *can*
get dropped and not handled.  This patch doesn't address this,
but just mitigates fallout caused by the current implemention.
Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -359,10 +359,12 @@ static enum skb_state defer_bh(struct usbnet *dev, struct sk_buff *skb,
 void usbnet_defer_kevent (struct usbnet *dev, int work)
 {
 	set_bit (work, &dev->flags);
-	if (!schedule_work (&dev->kevent))
+	if (!schedule_work (&dev->kevent)) {
+		if (net_ratelimit())
 			netdev_err(dev->net, "kevent %d may have been dropped\n", work);
-	else
+	} else {
 		netdev_dbg(dev->net, "kevent %d scheduled\n", work);
+	}
 }
 EXPORT_SYMBOL_GPL(usbnet_defer_kevent);