NTFS: 2.0.20 - Support non-resident directory index bitmaps, fix page leak in readdir.

NTFS: 2.0.20 - Support non-resident directory index bitmaps, fix page leak in readdir.
- Move the directory index bitmap to use an attribute inode instead of
  having special fields for it inside the ntfs inode structure. This
  means that the index bitmaps now use the page cache for i/o, too,
  and also as a side effect we get support for non-resident index
  bitmaps for free.
- Simplify/cleanup error handling in fs/ntfs/dir.c::ntfs_readdir() and
  fix a page leak that manifested itself in some cases.
- Add fs/ntfs/inode.c::ntfs_put_inode(), which we need to release the
  index bitmap inode on the final iput().