aufs: for v5.8-rc1, a_ops->readahead

simply follows
8151b4c8bee43 2020-06-02 mm: add readahead address space operation
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/dynop.c

@@ -167,6 +167,7 @@ static void dy_aop(struct au_dykey *key, const void *h_op,
 	DySetAop(writepages);
 	DySetAop(set_page_dirty);
 	DySetAop(readpages);
+	DySetAop(readahead);
 	DySetAop(write_begin);
 	DySetAop(write_end);
 	DySetAop(bmap);