- XFS has started to use clear_page_dirty(), so we should export
  test_clear_page_dirty() to modules.

  This function is ued by the inlined clear_page_dirty().  It marks a
  page clean and updates the global dirty memory accounting.  Anyone
  who cleans pagecache pages should use this, so the export makes
  sense.  Can't implement aops->writepages() without it, really.

- __mark_inode_dirty is no longer called under mapping->private_lock.
   Update comment.