drm/gem: WARN if drm_gem_get_pages is called on a private obj

No real functional change, since this just converts an annoying Oops
into a more harmless WARNING backtrace. It's still a driver bug.
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511093554.211493-3-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_gem.c

@@ -548,6 +548,10 @@ static void drm_gem_check_release_pagevec(struct pagevec *pvec)
  * set during initialization. If you have special zone constraints, set them
  * after drm_gem_object_init() via mapping_set_gfp_mask(). shmem-core takes care
  * to keep pages in the required zone during swap-in.
+ *
+ * This function is only valid on objects initialized with
+ * drm_gem_object_init(), but not for those initialized with
+ * drm_gem_private_object_init() only.
  */
 struct page **drm_gem_get_pages(struct drm_gem_object *obj)
 {
@@ -556,6 +560,10 @@ struct page **drm_gem_get_pages(struct drm_gem_object *obj)
 	struct pagevec pvec;
 	int i, npages;
 
+
+	if (WARN_ON(!obj->filp))
+		return ERR_PTR(-EINVAL);
+
 	/* This is the shared memory object that backs the GEM resource */
 	mapping = obj->filp->f_mapping;