mm: use pgdat_end_pfn() to simplify the code in arch

Use "pgdat_end_pfn()" instead of "pgdat->node_start_pfn +
pgdat->node_spanned_pages".  Simplify the code, no functional change.

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/ia64/mm/init.c

@@ -357,9 +357,7 @@ int vmemmap_find_next_valid_pfn(int node, int i)
 
 	end_address = (unsigned long) &vmem_map[pgdat->node_start_pfn + i];
 	end_address = PAGE_ALIGN(end_address);
-
-	stop_address = (unsigned long) &vmem_map[
-		pgdat->node_start_pfn + pgdat->node_spanned_pages];
+	stop_address = (unsigned long) &vmem_map[pgdat_end_pfn(pgdat)];
 
 	do {
 		pgd_t *pgd;