Force a netdevice poll in net_tx() before attempting to transmit

packet, to work around the problem whereby small TX rings get backed
up because we haven't yet poll()ed for TX completions.

src/net/netdevice.c

  */
 int net_tx ( struct io_buffer *iobuf, struct net_device *netdev,
 	     struct net_protocol *net_protocol, const void *ll_dest ) {
+
+	/* Force a poll on the netdevice to (potentially) clear any
+	 * backed-up TX completions.  This is needed on some network
+	 * devices to avoid excessive losses due to small TX ring
+	 * sizes.
+	 */
+	netdev_poll ( netdev );
+
 	return netdev->ll_protocol->tx ( iobuf, netdev, net_protocol, ll_dest );
 }