powerpc/mm: Use the correct pointer when setting a 2MB pte

When setting a 2MB pte, radix__map_kernel_page() is using the address

	ptep = (pte_t *)pudp;

Fix this conversion to use pmdp instead. Use pmdp_ptep() to do this
instead of casting the pointer.

Fixes: 2bfd65e4 ("powerpc/mm/radix: Add radix callbacks for early init routines")
Cc: stable@vger.kernel.org # v4.7+
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Reza Arbab <arbab@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>