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>