mm/page_alloc.c: use free_area_empty() instead of open-coding

Use free_area_empty() API to replace list_empty() for better code
readability.
Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: http://lkml.kernel.org/r/1583674354-7713-1-git-send-email-qiwuchen55@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3460,8 +3460,7 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 			return true;
 		}
 #endif
-		if (alloc_harder &&
-			!list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
+		if (alloc_harder && !free_area_empty(area, MIGRATE_HIGHATOMIC))
 			return true;
 	}
 	return false;