x86: use PTE_MASK in 32-bit PAE

Use PTE_MASK in 3-level pagetables (ie, 32-bit PAE).

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>