xen/grant-table: Make set/clear page private code shared

Make set/clear page private code shared and accessible to
other kernel modules which can re-use these instead of open-coding.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>