ext4: fix up remaining files with SPDX cleanups

A number of ext4 source files were skipped due because their copyright
permission statements didn't match the expected text used by the
automated conversion utilities.  I've added SPDX tags for the rest.

While looking at some of these files, I've noticed that we have quite
a bit of variation on the licenses that were used --- in particular
some of the Red Hat licenses on the jbd2 files use a GPL2+ license,
and we have some files that have a LGPL-2.1 license (which was quite
surprising).

I've not attempted to do any license changes.  Even if it is perfectly
legal to relicense to GPL 2.0-only for consistency's sake, that should
be done with ext4 developer community discussion.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/checkpoint.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * linux/fs/jbd2/checkpoint.c
  *
@@ -5,10 +6,6 @@
  *
  * Copyright 1999 Red Hat Software --- All Rights Reserved
  *
- * This file is part of the Linux kernel and is made available under
- * the terms of the GNU General Public License, version 2, or at your
- * option, any later version, incorporated herein by reference.
- *
  * Checkpoint routines for the generic filesystem journaling code.
  * Part of the ext2fs journaling system.
  *