x86: clarify use of _PAGE_CHG_MASK

_PAGE_CHG_MASK is defined as the set of bits not updated by
pte_modify(); specifically, the pfn itself, and the Accessed and Dirty
bits (which are updated by hardware).
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Tested-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-x86/pgtable.h

@@ -57,6 +57,7 @@
 #define _KERNPG_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED |	\
 			 _PAGE_DIRTY)
 
+/* Set of bits not changed in pte_modify */
 #define _PAGE_CHG_MASK	(PTE_MASK | _PAGE_PCD | _PAGE_PWT |		\
 			 _PAGE_ACCESSED | _PAGE_DIRTY)