vfio: use __aligned_u64 in struct vfio_device_gfx_plane_info

vfio: use __aligned_u64 in struct vfio_device_gfx_plane_info

The memory layout of struct vfio_device_gfx_plane_info is
architecture-dependent due to a u64 field and a struct size that is not
a multiple of 8 bytes:
- On x86_64 the struct size is padded to a multiple of 8 bytes.
- On x32 the struct size is only a multiple of 4 bytes, not 8.
- Other architectures may vary.

Use __aligned_u64 to make memory layout consistent. This reduces the
chance of 32-bit userspace on a 64-bit kernel breakage.

This patch increases the struct size on x32 but this is safe because of
the struct's argsz field. The kernel may grow the struct as long as it
still supports smaller argsz values from userspace (e.g. applications
compiled against older kernel headers).

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20230918205617.1478722-3-stefanha@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>