x86/cpa: use an undefined PTE bit for testing CPA

Rather than using _PAGE_GLOBAL - which not all CPUs support - to test
CPA, use one of the reserved-for-software-use PTE flags instead.  This
allows CPA testing to work on CPUs which don't support PGD.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>