ip_tunnel: use the right netns in ioctl handler

Because the netdevice may be in another netns than the i/o netns, we should
use the i/o netns instead of dev_net(dev).

The variable 'tunnel' was used only to get 'itn', hence to simplify code I
remove it and use 't' instead.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>