agp: info leak in agpioc_info_wrap()

On 64 bit systems the agp_info struct has a 4 byte hole between
->agp_mode and ->aper_base.  We need to clear it to avoid disclosing
stack information to userspace.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>