btrfs: replace GPL boilerplate by SPDX -- sources

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

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

fs/btrfs/ulist.c

@@ -1,7 +1,7 @@
+// 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.
  */
 
 #include <linux/slab.h>