powerpc/mm/hugetlb: Use pte_access_permitted for hugetlb access check

No functional change in this patch. This update gup_hugepte to use the
helper. This will help later when we add memory keys.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>