mm: remove unused zone_idx variable from set_migratetype_isolate

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -5502,10 +5502,8 @@ int set_migratetype_isolate(struct page *page)
 	struct memory_isolate_notify arg;
 	int notifier_ret;
 	int ret = -EBUSY;
-	int zone_idx;
 
 	zone = page_zone(page);
-	zone_idx = zone_idx(zone);
 
 	spin_lock_irqsave(&zone->lock, flags);