x86: use PTE_MASK rather than ad-hoc mask

Use ~PTE_MASK to extract the non-pfn parts of the pte (ie, the pte
flags), rather than constructing an ad-hoc mask.

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>