A forward-port.  This is the code which prevents ENOSPC
errors from exposing stale data within filesystems.

- in generic_file_write(), if prepare_write() fails, truncate
  the file to drop any part-added blocks.

- in __block_write_full_page(), if we hit an error,  push
  whatever buffers we _have_ mapped into the file out to disk.

- in __block_prepare_write(), if we hit an error, zero out
  any blocks which we did manage to map into the file.  This
  is because the caller won't be doing any writing to those
  blocks due to the error.