records in the page are dirty and if so redirties the page and
      returns.  Otherwise it just returns (after doing set_page_writeback(),
      unlock_page(), end_page_writeback() or the radix-tree tag
      PAGECACHE_TAG_DIRTY  remains set even though the page is clean), thus
      alowing the VM to do with the page as it pleases.  Also, at umount
      time, now only throw away dirty mft (meta)data pages if dirty inodes
      are present and ask the user to email us if they see this happening.
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>