- a few more header cleanups

- removed some unused code
- inserted many extra BUG()s to catch problems early
- fixed a use of the incorrect inode, and fixed the directory bug
- enabled mds_open/mds_close
- fixed a small leak in an error case
- removed an extra set_page_dirty() in the write path
- added an open/write/close/open/unlink/read/truncate/write/read/close test