net: add sysfs entry for device group

The group of a network device can be queried or changed from userspace
using sysfs.

For example, considering sysfs mounted in /sys, one can change the group
that interface lo belongs to:
	echo 1 > /sys/class/net/lo/group
Signed-off-by: Vlad Dogaru <ddvlad@rosedu.org>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net-sysfs.c

@@ -295,6 +295,20 @@ static ssize_t show_ifalias(struct device *dev,
 	return ret;
 }
 
+NETDEVICE_SHOW(group, fmt_dec);
+
+static int change_group(struct net_device *net, unsigned long new_group)
+{
+	dev_set_group(net, (int) new_group);
+	return 0;
+}
+
+static ssize_t store_group(struct device *dev, struct device_attribute *attr,
+			 const char *buf, size_t len)
+{
+	return netdev_store(dev, attr, buf, len, change_group);
+}
+
 static struct device_attribute net_class_attributes[] = {
 	__ATTR(addr_assign_type, S_IRUGO, show_addr_assign_type, NULL),
 	__ATTR(addr_len, S_IRUGO, show_addr_len, NULL),
@@ -316,6 +330,7 @@ static struct device_attribute net_class_attributes[] = {
 	__ATTR(flags, S_IRUGO | S_IWUSR, show_flags, store_flags),
 	__ATTR(tx_queue_len, S_IRUGO | S_IWUSR, show_tx_queue_len,
 	       store_tx_queue_len),
+	__ATTR(group, S_IRUGO | S_IWUSR, show_group, store_group),
 	{}
 };