fs: btrfs: add zstd decompression support

This adds decompression support for Zstandard, which has been included
in Linux btrfs driver for some time.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/fs/btrfs/btrfs_tree.h b/fs/btrfs/btrfs_tree.h
index f90fbb2..aa0f3d6 100644
--- a/fs/btrfs/btrfs_tree.h
+++ b/fs/btrfs/btrfs_tree.h
@@ -647,8 +647,9 @@
 	BTRFS_COMPRESS_NONE  = 0,
 	BTRFS_COMPRESS_ZLIB  = 1,
 	BTRFS_COMPRESS_LZO   = 2,
-	BTRFS_COMPRESS_TYPES = 2,
-	BTRFS_COMPRESS_LAST  = 3,
+	BTRFS_COMPRESS_ZSTD  = 3,
+	BTRFS_COMPRESS_TYPES = 3,
+	BTRFS_COMPRESS_LAST  = 4,
 };
 
 struct btrfs_file_extent_item {