I just tested this and it worked:
text_list = TextList.from_folder(path, vocab=data_lm.vocab)
text_list_split_by_folder = text_list.split_by_folder(valid='test')
text_list_labeled_by_folder = text_list_split_by_folder.label_from_folder(classes=['neg', 'pos'])
text_list_labeled_by_folder.filter_missing_y = True
data_clas = text_list_labeled_by_folder.databunch(bs=bs)
data_clas.save('tmp_clas')