drm: integer overflow in drm_mode_dirtyfb_ioctl()

There is a potential integer overflow in drm_mode_dirtyfb_ioctl()
if userspace passes in a large num_clips.  The call to kmalloc would
allocate a small buffer, and the call to fb->funcs->dirty may result
in a memory corruption.

Reported-by: Haogang Chen <haogangchen@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
void drm_crtc_cleanup(struct drm_crtc *crtc)
{
	struct drm_device *dev = crtc->dev;

	if (crtc->gamma_store) {
		kfree(crtc->gamma_store);
		crtc->gamma_store = NULL;
	}

	drm_mode_object_put(dev, &crtc->base);
	list_del(&crtc->head);
	dev->mode_config.num_crtc--;
}

void drm_crtc_cleanup(struct drm_crtc *crtc)
{
	struct drm_device *dev = crtc->dev;

	if (crtc->gamma_store) {
		kfree(crtc->gamma_store);
		crtc->gamma_store = NULL;
	}

	drm_mode_object_put(dev, &crtc->base);
	list_del(&crtc->head);
	dev->mode_config.num_crtc--;
}
