powerpc/mm: Fix vma_mmu_pagesize() for radix

Radix doesn't use the slice framework to find the page size. Hence use
vma to find the page size.

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