btrfs: define compression levels statically

The maximum and default levels do not change and can be defined
directly. The set_level callback was a temporary solution and will be
removed.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.h

@@ -162,6 +162,10 @@ struct btrfs_compress_op {
 	 * if the level is out of bounds or the default if 0 is passed in.
 	 */
 	unsigned int (*set_level)(unsigned int level);
+
+	/* Maximum level supported by the compression algorithm */
+	unsigned int max_level;
+	unsigned int default_level;
 };
 
 /* The heuristic workspaces are managed via the 0th workspace manager */