powerpc/mm: Return NULL for not present hugetlb page

We need to check whether pte is present in follow_huge_addr() and
properly return NULL if mapping is not present. Also use READ_ONCE
when dereferencing pte_t address.

Without this patch, we may wrongly return a zero pfn page in
follow_huge_addr().

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