mm: optimize pfn calculation in online_page()

If CONFIG_FLATMEM is enabled pfn is calculated in online_page() more than
once.  It is possible to optimize that and use value established at
beginning of that function.
Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -375,7 +375,7 @@ void online_page(struct page *page)
 #endif
 
 #ifdef CONFIG_FLATMEM
-	max_mapnr = max(page_to_pfn(page), max_mapnr);
+	max_mapnr = max(pfn, max_mapnr);
 #endif
 
 	ClearPageReserved(page);