ext4: fix prefetchw of NULL page

In ext4_mpage_readpages(), if the parameter pages is not NULL, another
parameter page is NULL. At the first time prefetchw(&page->flags)
works on NULL. From second time, prefetchw(&page->flags) always works on
the last consumed page. This might do little improvment for handling
current page. So prefetchw() should be called while the page pointer
has just been updated.
Signed-off-by: Liu Xiang <liu.xiang6@zte.com.cn>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/readpage.c

@@ -127,9 +127,10 @@ int ext4_mpage_readpages(struct address_space *mapping,
 		int fully_mapped = 1;
 		unsigned first_hole = blocks_per_page;
 
-		prefetchw(&page->flags);
 		if (pages) {
 			page = lru_to_page(pages);
+
+			prefetchw(&page->flags);
 			list_del(&page->lru);
 			if (add_to_page_cache_lru(page, mapping, page->index,
 				  readahead_gfp_mask(mapping)))