Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/um/drivers/ubd_user.c b/arch/um/drivers/ubd_user.c
index 6f74479..a1afe41 100644
--- a/arch/um/drivers/ubd_user.c
+++ b/arch/um/drivers/ubd_user.c
@@ -1,8 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2016 Anton Ivanov (aivanov@brocade.com)
  * Copyright (C) 2000, 2001, 2002 Jeff Dike (jdike@karaya.com)
  * Copyright (C) 2001 Ridgerun,Inc (glonnon@ridgerun.com)
- * Licensed under the GPL
  */
 
 #include <stddef.h>