drm: prevent a harmless integer overflow in drm_legacy_sg_alloc()

There is an integer overflow when we round up to PAGE_SIZE, but it's
harmless because we never re-use "request->size" for anything meaningful.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228092321.axulddmkxrujkmas@kili.mountain

drivers/gpu/drm/drm_scatter.c

@@ -99,6 +99,9 @@ int drm_legacy_sg_alloc(struct drm_device *dev, void *data,
 	if (!drm_core_check_feature(dev, DRIVER_SG))
 		return -EOPNOTSUPP;
 
+	if (request->size > SIZE_MAX - PAGE_SIZE)
+		return -EINVAL;
+
 	if (dev->sg)
 		return -EINVAL;