powerpc/mm/radix: Add THP support for 4K linux page size

This adds THP support for 4K Linux page size config with radix. We still
don't do THP with 4K Linux page size and hash page table. Hash page
table needs a 16MB hugepage and we can't do THP with 16MM hugepage and
4K Linux page size.

We add missing functions to 4K hash config to get it to build and
hash__has_transparent_hugepage() makes sure we don't enable THP for 4K
hash config. To catch wrong usage of THP related with 4K config, we add
BUG() in those dummy functions we added to get it compile.

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