readahead: apply max_sane_readahead() limit in ondemand_readahead()

Just in case someone aggressively sets a huge readahead size.

Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/readahead.c

@@ -357,7 +357,7 @@ ondemand_readahead(struct address_space *mapping,
 		   bool hit_readahead_marker, pgoff_t offset,
 		   unsigned long req_size)
 {
-	int	max = ra->ra_pages;	/* max readahead pages */
+	unsigned long max = max_sane_readahead(ra->ra_pages);
 	pgoff_t prev_offset;
 	int	sequential;