TODO

* cleanup, add more error checking, get rid of BUG_ONs
* Fix ENOSPC handling
* Make allocator smarter
* add a block group to struct inode
* Do actual block accounting
* Check compat and incompat flags on the inode
* Get rid of struct ctree_path, limiting tree levels held at one time
* Add generation number to key pointer in nodes
* Add generation number to inode
* forbid cross subvolume renames and hardlinks
* Release
* Do real tree locking
* Add extent mirroring (backup copies of blocks)
* Add fancy interface to get access to incremental backups
* Add fancy striped extents to make big reads faster
* Use relocation to try and fix write errors
* Make allocator much smarter
* xattrs (directory streams for regular files)
* Scrub & defrag