drm/i915: fix integer overflow in i915_gem_do_execbuffer()

drm/i915: fix integer overflow in i915_gem_do_execbuffer()

commit 44afb3a04391a74309d16180d1e4f8386fdfa745 upstream.

On 32-bit systems, a large args->num_cliprects from userspace via ioctl
may overflow the allocation size, leading to out-of-bounds access.

This vulnerability was introduced in commit 432e58ed ("drm/i915: Avoid
allocation for execbuffer object list").

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>