powerpc/mm/radix: Use the right page size for vmemmap mapping

We use mmu_vmemmap_psize to find the page size for mapping the vmmemap area.
With radix translation, we are suboptimally setting this value to PAGE_SIZE.

We do check for 2M page size support and update mmu_vmemap_psize to use
hugepage size but we suboptimally reset the value to PAGE_SIZE in
radix__early_init_mmu(). This resulted in always mapping vmemmap area with
64K page size.

Fixes: 2bfd65e4 ("powerpc/mm/radix: Add radix callbacks for early init routines")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>