drm/i915: fix integer overflow in i915_gem_do_execbuffer()

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>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
eb_add_object(struct eb_objects *eb, struct drm_i915_gem_object *obj)
{
	hlist_add_head(&obj->exec_node,
		       &eb->buckets[obj->exec_handle & eb->and]);
}

eb_add_object(struct eb_objects *eb, struct drm_i915_gem_object *obj)
{
	hlist_add_head(&obj->exec_node,
		       &eb->buckets[obj->exec_handle & eb->and]);
}
