tun: Implement ethtool's get_channels() callback

Implement the tun .get_channels functionality. This feature is necessary
for some tools, such as libxdp, which need to retrieve the queue count.

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>