macvtap: update file current position

[ Upstream commit e6ebc7f16ca1434a334647aa56399c546be4e64b ]

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>