Btrfs: Make the FS tree the last objectid in the tree of tree roots

/* stores information about which extents are in use, and reference counts */

#define BTRFS_EXTENT_TREE_OBJECTID 2ULL

/* one per subvolume, storing files and directories */

#define BTRFS_FS_TREE_OBJECTID 3ULL

/* directory objectid inside the root tree */

#define BTRFS_ROOT_TREE_DIR_OBJECTID 4ULL

/*

 * chunk tree stores translations from logical -> physical block numbering

 * the super block points to the chunk tree

 */

#define BTRFS_CHUNK_TREE_OBJECTID 5ULL

#define BTRFS_CHUNK_TREE_OBJECTID 3ULL

/*

 * stores information about which areas of a given device are in use.

 * one per device.  The tree of tree roots points to the device tree

 */

#define BTRFS_DEV_TREE_OBJECTID 6ULL

#define BTRFS_DEV_TREE_OBJECTID 4ULL

/* one per subvolume, storing files and directories */

#define BTRFS_FS_TREE_OBJECTID 5ULL

/* directory objectid inside the root tree */

#define BTRFS_ROOT_TREE_DIR_OBJECTID 6ULL

/*

 * All files have objectids higher than this.