vfio: use __aligned_u64 in struct vfio_device_ioeventfd

vfio: use __aligned_u64 in struct vfio_device_ioeventfd

The memory layout of struct vfio_device_ioeventfd 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 that 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).

The code that uses struct vfio_device_ioeventfd already works correctly
when the struct size grows, so only the struct definition needs to be
changed.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20230918205617.1478722-4-stefanha@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>