mm, memory hotplug: small cleanup in online_pages()

We can reuse the nid we've determined instead of repeated pfn_to_nid()
usages.  Also zone_to_nid() should be a bit cheaper in general than
pfn_to_nid().
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -1055,7 +1055,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
 	arg.nr_pages = nr_pages;
 	node_states_check_changes_online(nr_pages, zone, &arg);
 
-	nid = pfn_to_nid(pfn);
+	nid = zone_to_nid(zone);
 
 	ret = memory_notify(MEM_GOING_ONLINE, &arg);
 	ret = notifier_to_errno(ret);
@@ -1095,7 +1095,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
 	pgdat_resize_unlock(zone->zone_pgdat, &flags);
 
 	if (onlined_pages) {
-		node_states_set_node(zone_to_nid(zone), &arg);
+		node_states_set_node(nid, &arg);
 		if (need_zonelists_rebuild)
 			build_all_zonelists(NULL, NULL);
 		else
@@ -1107,7 +1107,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
 	init_per_zone_wmark_min();
 
 	if (onlined_pages) {
-		kswapd_run(zone_to_nid(zone));
+		kswapd_run(nid);
 		kcompactd_run(nid);
 	}