powerpc/mm/hash64: Allocate larger PMD table if hugetlb config is enabled

We use the second half of the page table to store slot information, so we must
allocate it always if hugetlb is possible.

Fixes: bf9a95f9

 ("powerpc: Free up four 64K PTE bits in 64K backed HPTE pages")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>