storage: move check for convert_images to GetDiffer

move the check for `convert_images` to GetDiffer so that it doesn't
attempt any operation if the feature is disabled.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/chunked/storage_linux.go

@@ -241,6 +241,10 @@ func GetDiffer(ctx context.Context, store storage.Store, blobDigest digest.Diges
 		return nil, err
 	}
 
+	if !parseBooleanPullOption(&storeOpts, "convert_images", false) {
+		return nil, errors.New("convert_images not configured")
+	}
+
 	_, hasZstdChunkedTOC := annotations[internal.ManifestChecksumKey]
 	_, hasEstargzTOC := annotations[estargz.TOCJSONDigestAnnotation]
 
@@ -259,10 +263,6 @@ func GetDiffer(ctx context.Context, store storage.Store, blobDigest digest.Diges
 }
 
 func makeConvertFromRawDiffer(ctx context.Context, store storage.Store, blobDigest digest.Digest, blobSize int64, annotations map[string]string, iss ImageSourceSeekable, storeOpts *types.StoreOptions) (*chunkedDiffer, error) {
-	if !parseBooleanPullOption(storeOpts, "convert_images", false) {
-		return nil, errors.New("convert_images not configured")
-	}
-
 	layersCache, err := getLayersCache(store)
 	if err != nil {
 		return nil, err