tun: fix carrier on/off status

Commit c8d68e6b removed carrier off call
from tun_detach since it's now called on queue disable and not only on
tun close.  This confuses userspace which used this flag to detect a
free tun. To fix, put this back but under if (clean).

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>