btrfs: replace GPL boilerplate by SPDX -- headers

Remove GPL boilerplate text (long, short, one-line) and keep the rest,
ie. personal, company or original source copyright statements. Add the
SPDX header.

Unify the include protection macros to match the file names.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ulist.h

@@ -1,12 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2011 STRATO AG
  * written by Arne Jansen <sensille@gmx.net>
- * Distributed under the GNU GPL license version 2.
- *
  */
 
-#ifndef __ULIST__
-#define __ULIST__
+#ifndef BTRFS_ULIST_H
+#define BTRFS_ULIST_H
 
 #include <linux/list.h>
 #include <linux/rbtree.h>