memory-hotplug: don't calculate vm_total_pages twice when rebuilding zonelists in online_pages()

If zonelist is required to be rebuilt in online_pages(), there is no need
to recalculate vm_total_pages in that function, as it has been updated in
the call build_all_zonelists().
Signed-off-by: Kent Liu <kent.liu@linux.intel.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -429,7 +429,9 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 
 	if (need_zonelists_rebuild)
 		build_all_zonelists();
-	vm_total_pages = nr_free_pagecache_pages();
+	else
+		vm_total_pages = nr_free_pagecache_pages();
+
 	writeback_set_ratelimit();
 
 	if (onlined_pages)