mm: Export writeback_iter()

Export writeback_iter() so that it can be used by netfslib as a module.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox (Oracle) <willy@infradead.org>
cc: Christoph Hellwig <hch@lst.de>
cc: linux-mm@kvack.org

mm/page-writeback.c

@@ -2546,6 +2546,7 @@ struct folio *writeback_iter(struct address_space *mapping,
 	folio_batch_release(&wbc->fbatch);
 	return NULL;
 }
+EXPORT_SYMBOL_GPL(writeback_iter);
 
 /**
  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.