net: allow to delete a whole device group

With dev group, we can change a batch of net devices,
so we should allow to delete them together too.

Group 0 is not allowed to be deleted since it is
the default group.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>