mm: remove unneeded local variable in free_area_init_core

Local variable 'zone_start_pfn' is not needed since there's only one call
site in free_area_init_core().  Let's remove it and pass
zone->zone_start_pfn directly to init_currently_empty_zone().

Link: https://lkml.kernel.org/r/20210122135956.5946-6-bhe@redhat.comSigned-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -6927,7 +6927,6 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
 	for (j = 0; j < MAX_NR_ZONES; j++) {
 		struct zone *zone = pgdat->node_zones + j;
 		unsigned long size, freesize, memmap_pages;
-		unsigned long zone_start_pfn = zone->zone_start_pfn;
 
 		size = zone->spanned_pages;
 		freesize = zone->present_pages;
@@ -6976,7 +6975,7 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
 
 		set_pageblock_order();
 		setup_usemap(zone);
-		init_currently_empty_zone(zone, zone_start_pfn, size);
+		init_currently_empty_zone(zone, zone->zone_start_pfn, size);
 		memmap_init_zone(zone);
 	}
 }