[PATCH] check for presence of readpage() in the readahead code

Make sure that the address_space is capable of performing the readahead
before going in and allocating the pages.

mm/readahead.c

@@ -260,6 +260,9 @@ int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 {
 	int ret = 0;
 
+	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
+		return -EINVAL;
+
 	while (nr_to_read) {
 		int err;