tun: avoid owner checks on IFF_ATTACH_QUEUE

At the moment, we check owner when we enable queue in tun.
This seems redundant and will break some valid uses
where fd is passed around: I think TUNSETOWNER is there
to prevent others from attaching to a persistent device not
owned by them. Here the fd is already attached,
enabling/disabling queue is more like read/write.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>