net: rename group sysfs entry to netdev_group

commit a512b92b

 adds sysfs entry for net device group, but
before this commit, tun also uses group sysfs, so after this
commit checkin, kernel warns like this:
    sysfs: cannot create duplicate filename '/devices/virtual/net/vnet0/group'

Since tun has used this for years, rename sysfs under tun might
break existing userspace, so rename group sysfs entry for net device
group is a better choice.

Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>