mm: update the comment in __isolate_free_page

We need to assure the comment is consistent with the code.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/1466171914-21027-1-git-send-email-zhongjiang@huawei.com
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2517,7 +2517,10 @@ int __isolate_free_page(struct page *page, unsigned int order)
 	zone->free_area[order].nr_free--;
 	rmv_page_order(page);
 
-	/* Set the pageblock if the isolated page is at least a pageblock */
+	/*
+	 * Set the pageblock if the isolated page is at least half of a
+	 * pageblock
+	 */
 	if (order >= pageblock_order - 1) {
 		struct page *endpage = page + (1 << order) - 1;
 		for (; page < endpage; page += pageblock_nr_pages) {