drm/vc4: Add a pad field to align drm_vc4_submit_cl to 64 bits.

drm/vc4: Add a pad field to align drm_vc4_submit_cl to 64 bits.

I had originally asked Stefan Schake to drop the pad field from the
syncobj changes that just landed, because I couldn't come up with a
reason to align to 64 bits.

Talking with Dave Airlie about the new v3d driver's submit ioctl, we
came up with a reason: sizeof() on 64-bit platforms may align to 64
bits, in which case the userspace will be submitting the aligned size
and the final 32 bits won't be zero-padded by the kernel.  If
userspace doesn't zero-fill, then a future ABI change adding a 32-bit
field at the end could potentially cause the kernel to read undefined
data from old userspace (our userspace happens to use structure
initialization that zero-fills, but as a general rule we try not to
rely on that in the kernel).

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180430235927.28712-1-eric@anholt.net
Reviewed-by: Stefan Schake <stschake@gmail.com>