[PATCH] fix layout of mapped files

If you create a shared mapping of a sparse file, dirty it
and then run msync, all the file's blocks are laid out
backwards.

This is because filemap_sync puts the lowest-index page at
mapping->dirty_pages.prev and the highest at mapping->dirty_pages.next.

I think that by walking the dirty pages list in ascending file
offset order as we instantiate their disk mappings we will generally
get better layout.

mm/filemap.c

@@ -550,7 +550,7 @@ int filemap_fdatasync(struct address_space * mapping)
 	spin_lock(&pagecache_lock);
 
         while (!list_empty(&mapping->dirty_pages)) {
-		struct page *page = list_entry(mapping->dirty_pages.next, struct page, list);
+		struct page *page = list_entry(mapping->dirty_pages.prev, struct page, list);
 
 		list_del(&page->list);
 		list_add(&page->list, &mapping->locked_pages);