Add sync mode for 'xl.json' (#6798)

xl.json is the source of truth for all erasure
coded objects, without which we won't be able to
read the objects properly. This PR enables sync
mode for writing `xl.json` such all writes go hit
the disk and are persistent under situations such
as abrupt power failures on servers running Minio.

cmd/xl-v1-metadata.go

@@ -423,8 +423,9 @@ func writeXLMetadata(ctx context.Context, disk StorageAPI, bucket, prefix string
 		logger.LogIf(ctx, err)
 		return err
 	}
+
 	// Persist marshaled data.
-	err = disk.AppendFile(bucket, jsonFile, metadataBytes)
+	err = disk.WriteAll(bucket, jsonFile, metadataBytes)
 	logger.LogIf(ctx, err)
 	return err
 }