rbd: plug rbd_dev->header.object_prefix memory leak

Need to free object_prefix when rbd_dev_v2_snap_context() fails, but
only if this is the first time we are reading in the header.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -4720,7 +4720,10 @@ static int rbd_dev_v2_header_info(struct rbd_device *rbd_dev)
 	}
 
 	ret = rbd_dev_v2_snap_context(rbd_dev);
-	dout("rbd_dev_v2_snap_context returned %d\n", ret);
+	if (ret && first_time) {
+		kfree(rbd_dev->header.object_prefix);
+		rbd_dev->header.object_prefix = NULL;
+	}
 
 	return ret;
 }