Allow disabling the tx checksums on the xen loopback

interface, which is used for the virtual interfaces of dom0 instead 
of netfront/netback.

I need to issue ethtool -K eth0 tx off to work around a bad checksum
problem, which has been reported for domUs a few months ago. I'm
bridging dom0's eth0 to a domU where I do NAT to a different virtual
interface. Without this change, I get lots of packets with bad
checksums.

Signed-off-by: Jan Niehusmann <jan@gondor.com>

linux-2.6-xen-sparse/drivers/xen/netback/loopback.c

@@ -27,6 +27,7 @@
 #include <linux/inetdevice.h>
 #include <linux/etherdevice.h>
 #include <linux/skbuff.h>
+#include <linux/ethtool.h>
 #include <net/dst.h>
 
 static int nloopbacks = 8;
@@ -122,6 +123,12 @@ static void loopback_construct(struct net_device *dev, struct net_device *lo)
 	/*dev->mtu             = 16*1024;*/
 }
 
+static struct ethtool_ops network_ethtool_ops =
+{
+	.get_tx_csum = ethtool_op_get_tx_csum,
+	.set_tx_csum = ethtool_op_set_tx_csum,
+};
+
 static int __init make_loopback(int i)
 {
 	struct net_device *dev1, *dev2;
@@ -141,6 +148,8 @@ static int __init make_loopback(int i)
 	dev1->features |= NETIF_F_NO_CSUM;
 	dev2->features |= NETIF_F_IP_CSUM;
 
+	SET_ETHTOOL_OPS(dev2, &network_ethtool_ops);
+
 	/*
 	 * Initialise a dummy MAC address for the 'dummy backend' interface. We
 	 * choose the numerically largest non-broadcast address to prevent the