instead of void and provide a helper ntfs_truncate_vfs() for the
        vfs ->truncate method.
      - Add a new ntfs inode flag NInoTruncateFailed() and modify
        fs/ntfs/inode.c::ntfs_truncate() to set and clear it appropriately.
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>