mm: page_alloc: use get_freepage_migratetype() instead of page_private()

The page allocator uses set_page_private and page_private for handling
migratetype when it frees page.  Let's replace them with [set|get]
_freepage_migratetype to make it more clear.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 345643b..9c03dca 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -203,7 +203,7 @@
 		if (PageBuddy(page))
 			pfn += 1 << page_order(page);
 		else if (page_count(page) == 0 &&
-				page_private(page) == MIGRATE_ISOLATE)
+			get_freepage_migratetype(page) == MIGRATE_ISOLATE)
 			pfn += 1;
 		else
 			break;