powerpc/mm/radix: Make sure swapper pgdir is properly aligned

With 4K page size radix config our level 1 page table size is 64K and it
should be naturally aligned.

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