file operations ->write(), ->aio_write(), and ->writev() for regular
      files.  This replaces the old use of generic_file_write(), et al and
      the address space operations ->prepare_write and ->commit_write.
      This means that both sparse and non-sparse (unencrypted and
      uncompressed) files can now be extended using the normal write(2)
      code path.  There are two limitations at present and these are that
      we never create sparse files and that we only have limited support
      for highly fragmented files, i.e. ones whose data attribute is split
      across multiple extents.   When such a case is encountered,
      EOPNOTSUPP is returned.

Signed-off-by: Anton Altaparmakov <aia21@cantab.net>