drm/shmem-helper: Fix obj->filp derefence

I broke that in my refactoring:

commit 7d2cd72a
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri May 29 16:05:42 2020 +0200

    drm/shmem-helpers: Simplify dma-buf importing
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 7d2cd72a ("drm/shmem-helpers: Simplify dma-buf importing")
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200615151026.2339113-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -70,15 +70,17 @@ __drm_gem_shmem_create(struct drm_device *dev, size_t size, bool private)
 	mutex_init(&shmem->vmap_lock);
 	INIT_LIST_HEAD(&shmem->madv_list);
 
-	/*
-	 * Our buffers are kept pinned, so allocating them
-	 * from the MOVABLE zone is a really bad idea, and
-	 * conflicts with CMA. See comments above new_inode()
-	 * why this is required _and_ expected if you're
-	 * going to pin these pages.
-	 */
-	mapping_set_gfp_mask(obj->filp->f_mapping, GFP_HIGHUSER |
-			     __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
+	if (!private) {
+		/*
+		 * Our buffers are kept pinned, so allocating them
+		 * from the MOVABLE zone is a really bad idea, and
+		 * conflicts with CMA. See comments above new_inode()
+		 * why this is required _and_ expected if you're
+		 * going to pin these pages.
+		 */
+		mapping_set_gfp_mask(obj->filp->f_mapping, GFP_HIGHUSER |
+				     __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
+	}
 
 	return shmem;