- Add configuration option for developmental write support with an
  appropriately scary configuration help text.
- Initial implementation of fs/ntfs/aops.c::ntfs_writepage() and its
  helper fs/ntfs/aops.c::ntfs_write_block(). This enables mmap(2) based
  overwriting of existing files on ntfs. Note: Resident files are
  only written into memory, and not written out to disk at present, so
  avoid writing to files smaller than about 1kiB.
- Initial implementation of fs/ntfs/aops.c::ntfs_prepare_write(), its
  helper fs/ntfs/aops.c::ntfs_prepare_nonresident_write() and their
  counterparts, fs/ntfs/aops.c::ntfs_commit_write(), and
  fs/ntfs/aops.c::ntfs_commit_nonresident_write(), respectively. Also,
  add generic_file_write() to the ntfs file operations (fs/ntfs/file.c).
  This enables write(2) based overwriting of existing files on ntfs.
  Note: As with mmap(2) based overwriting, resident files are only
  written into memory, and not written out to disk at present, so avoid
  writing to files smaller than about 1kiB.
- Implement ->truncate (fs/ntfs/inode.c::ntfs_truncate()) and
  ->setattr() (fs/ntfs/inode.c::ntfs_setattr()) inode operations for
  files with the purpose of intercepting and aborting all i_size
  changes which we do not support yet. ntfs_truncate() actually only
  emits a warning message but AFAICS our interception of i_size changes
  elsewhere means ntfs_truncate() never gets called for i_size changes.
  It is only called from generic_file_write() when we fail in
  ntfs_prepare_{,nonresident_}write() in order to discard any
  instantiated buffers beyond i_size. Thus i_size is not actually
  changed so our warning message is enough. Unfortunately it is not
  possible to easily determine if i_size is being changed or not hence
  we just emit an appropriately worded error message.