x86: fix kernel_physical_mapping_init() for large x86 systems

Fix bug in kernel_physical_mapping_init() that causes kernel
page table to be built incorrectly for systems with greater
than 512GB of memory.
Signed-off-by: NJack Steiner <steiner@sgi.com>
Cc: linux-mm@kvack.org
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/init_64.c

@@ -644,7 +644,7 @@ static unsigned long __init kernel_physical_mapping_init(unsigned long start,
 		unsigned long pud_phys;
 		pud_t *pud;
 
-		next = start + PGDIR_SIZE;
+		next = (start + PGDIR_SIZE) & PGDIR_MASK;
 		if (next > end)
 			next = end;