- Make sure we have a valid inode when deleting a dentry
- use list_for_each_safe instead of manually walking ->d_subdirs
- don't do dget() in driverfs_mknod, since we already have a reference to it via driverfs_create_*
- so, remove extra dput() in driverfs_remove_dir and driverfs_remove_file
- Don't do get_mount() in driverfs_create_file, since it was done for the directory
- so remove extrra put_mount() from driverfs_remove_*
- Call d_invalidate() to unhash the files and directories instead of d_delete
- Remove our own d_unhash(), since d_invalidate() does the equivalent
- inline driverfs_rmdir in driverfs_remove_dir since its the only user and it prevents us from
  taking the dir's semaphore twice.