[PATCH] NFS: Fix O_DIRECT code

  - Support synchronous directio only. Defer asynchronous directio until
    it can be made safe.
  - If read/write exits due to an error, return number of bytes read/written
    prior to occurrence of the error.
  - Make sure we mark read pages as dirty in case we're doing zero-copy tricks.
    Export set_page_dirty_lock() for use by NFS directio.
  - Ensure we revalidate stale attribute info.

mm/page-writeback.c

@@ -549,6 +549,7 @@ int set_page_dirty_lock(struct page *page)
 	unlock_page(page);
 	return ret;
 }
+EXPORT_SYMBOL(set_page_dirty_lock);
 
 /*
  * Clear a page's dirty flag, while caring for dirty memory accounting.