tun: update file current position

[ Upstream commit d0b7da8afa079ffe018ab3e92879b7138977fc8f ]

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>