tun: fix carrier on/off status

tun: fix carrier on/off status

Commit c8d68e6be1c3b242f1c598595830890b65cea64a 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 <[email protected]>
Tested-by: Jason Wang <[email protected]>
Acked-by: Jason Wang <[email protected]>
Tested-by: Toralf Förster <[email protected]>
Signed-off-by: David S. Miller <[email protected]>