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>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 47a8fe9..fb32394 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
+
 #include <linux/bitops.h>
 #include <linux/slab.h>
 #include <linux/bio.h>