f2fs: add SPDX license identifiers

Remove the verbose license text from f2fs files and replace them with
SPDX tags.  This does not change the license of any of the code.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c
index a70cd25..904ad7b 100644
--- a/fs/f2fs/extent_cache.c
+++ b/fs/f2fs/extent_cache.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * f2fs extent cache support
  *
@@ -5,10 +6,6 @@
  * Copyright (c) 2015 Samsung Electronics
  * Authors: Jaegeuk Kim <jaegeuk@kernel.org>
  *          Chao Yu <chao2.yu@samsung.com>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
  */
 
 #include <linux/fs.h>