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>