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/extent_map.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-#ifndef __EXTENTMAP__
-#define __EXTENTMAP__
+
+#ifndef BTRFS_EXTENT_MAP_H
+#define BTRFS_EXTENT_MAP_H
 
 #include <linux/rbtree.h>
 #include <linux/refcount.h>
@@ -93,4 +94,5 @@ struct extent_map *search_extent_mapping(struct extent_map_tree *tree,
 					 u64 start, u64 len);
 int btrfs_add_extent_mapping(struct extent_map_tree *em_tree,
 			     struct extent_map **em_in, u64 start, u64 len);
+
 #endif