Unserialize ll_rmdir.

This unserializes all rmdir paths as a side effect.

Notes:

 * it is now clear that COND_ILOCK/IUNLOCK are problematic, esp
   in put_inode, will refactor in a later commit

 * it seems clear that Dir needs a mutex, will refactor in a
   later commit

Signed-off-by: Matt Benjamin <matt@linuxbox.com>