powerpc/mm: Change the swap encoding in pte.

Current swap encoding in pte can't support large pfns
above 4TB. Change the swap encoding such that we put
the swap type in the PTE bits. Also add build checks
to make sure we don't overlap with HPTEFLAGS.

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