- Remove some dead code from mft.c.
- Optimize readpage and read_block functions throughout aops.c so that
  only initialized blocks are read. Non-initialized ones have their
  buffer head mapped, zeroed, and set up to date, without scheduling
  any i/o. Thanks to Al Viro for advice on how to avoid the device i/o.
Thanks go to Andrew Morton for spotting the below:
- Fix buglet in allocate_compression_buffers() error code path.
- Call flush_dcache_page() after modifying page cache page contents in
  ntfs_file_readpage().
- Check for existence of page buffers throughout aops.c before calling
  create_empty_buffers(). This happens when an I/O error occurs and the
  read is retried. (It also happens once writing is implemented so that
  needed doing anyway but I had left it for later...)
- Don't BUG_ON() uptodate and/or mapped buffers throughout aops.c in
  readpage and read_block functions. Reasoning same as above (i.e. I/O
  error retries and future write code paths.)