[PATCH] export test_clear_page_dirty() to modules.

- 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.

mm/filemap.c

@@ -60,7 +60,7 @@
  *      ->swap_list_lock
  *        ->swap_device_lock	(exclusive_swap_page, others)
  *          ->mapping->page_lock
- *      ->inode_lock		(__mark_inode_dirty)
+ *  ->inode_lock
  *    ->sb_lock			(fs/fs-writeback.c)
  */
 

mm/page-writeback.c

@@ -601,3 +601,4 @@ int test_clear_page_dirty(struct page *page)
 	}
 	return 0;
 }
+EXPORT_SYMBOL(test_clear_page_dirty);