tuntap: forbid calling TUNSETIFF when detached

Michael points out that even after Stefan's fix the TUNSETIFF is still allowed
to create a new tap device. This because we only check tfile->tun but the
tfile->detached were introduced. Fix this by failing early in tun_set_iff() if
the file is detached. After this fix, there's no need to do the check again in
tun_set_iff(), so this patch removes it.

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