[PATCH] use page_to_pfn() in __blk_queue_bounce()

From: William Lee Irwin III <wli@holomorphy.com>

__blk_queue_bounce() contains an open-coded page_to_pfn() for the
discontig, non-MAP_NR_DENSE() case (wherever MAP_NR_DENSE() went).
This converts it to use the standard page_to_pfn() abstraction.

mm/highmem.c

@@ -381,7 +381,7 @@ static void __blk_queue_bounce(request_queue_t *q, struct bio **bio_orig,
 		/*
 		 * is destination page below bounce pfn?
 		 */
-		if ((page - page_zone(page)->zone_mem_map) + (page_zone(page)->zone_start_pfn) < q->bounce_pfn)
+		if (page_to_pfn(page) < q->bounce_pfn)
 			continue;
 
 		/*