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>
diff --git a/fs/btrfs/sysfs.h b/fs/btrfs/sysfs.h
index 80457f3..b567560 100644
--- a/fs/btrfs/sysfs.h
+++ b/fs/btrfs/sysfs.h
@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _BTRFS_SYSFS_H_
-#define _BTRFS_SYSFS_H_
+
+#ifndef BTRFS_SYSFS_H
+#define BTRFS_SYSFS_H
 
 /*
  * Data exported through sysfs
@@ -90,4 +91,4 @@ void btrfs_sysfs_remove_fsid(struct btrfs_fs_devices *fs_devs);
 void btrfs_sysfs_feature_update(struct btrfs_fs_info *fs_info,
 		u64 bit, enum btrfs_feature_set set);
 
-#endif /* _BTRFS_SYSFS_H_ */
+#endif