net: rtnl: info leak in rtnl_fill_vfinfo()

The "vf_vlan_info" struct ends with a 2 byte struct hole so we have to
memset it to ensure that no stack information is revealed to user space.

Fixes: 79aab093

 ('net: Update API for VF vlan protocol 802.1ad support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>