- Opening a hugetlbfs file O_TRUNC calls the generic vmtruncate() functions
  and nukes the kernel.

  Give S_ISREG hugetlbfs files a inode_operations, and hence a setattr
  which know how to handle these files.

- Don't permit the user to truncate hugetlbfs files to sizes which are not
  a multiple of HPAGE_SIZE.

- We don't support expanding in ftruncate(), so remove that code.