Use correct PUD for memory hotadd

From: Jan Beulich <jbeulich@novell.com>

Memory >39bits has a different PUD.

Cc: "Tolentino, Matthew E" <matthew.e.tolentino@intel.com>

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/arch/x86_64/mm/init.c
===================================================================
--- linux.orig/arch/x86_64/mm/init.c
+++ linux/arch/x86_64/mm/init.c
@@ -344,7 +344,7 @@ void __meminit init_memory_mapping(unsig
 		pud_t *pud;
 
 		if (after_bootmem)
-			pud = pud_offset_k(pgd, __PAGE_OFFSET);
+			pud = pud_offset_k(pgd, start & PGDIR_MASK);
 		else
 			pud = alloc_low_page(&map, &pud_phys);