powerpc/mm: Fix _PAGE_CHG_MASK to protect _PAGE_SPECIAL

Fix _PAGE_CHG_MASK so that pte_modify() does not affect the _PAGE_SPECIAL bit.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/include/asm/pgtable-64k.h b/arch/powerpc/include/asm/pgtable-64k.h
index 07b0d8f..7389003 100644
--- a/arch/powerpc/include/asm/pgtable-64k.h
+++ b/arch/powerpc/include/asm/pgtable-64k.h
@@ -114,7 +114,7 @@
  * pgprot changes
  */
 #define _PAGE_CHG_MASK	(PTE_RPN_MASK | _PAGE_HPTEFLAGS | _PAGE_DIRTY | \
-                         _PAGE_ACCESSED)
+                         _PAGE_ACCESSED | _PAGE_SPECIAL)
 
 /* Bits to mask out from a PMD to get to the PTE page */
 #define PMD_MASKED_BITS		0x1ff