net: Simplify loopback and improve batching.

Defer calling unregister_netdevice_queue to cleanup_net.  It's simpler
and it allows the loopback device to land in the same batch as other
network devices.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>