powerpc/mm/hugetlb: Don't enable HugeTLB if we don't have a page table cache

powerpc/mm/hugetlb: Don't enable HugeTLB if we don't have a page table cache

This makes sure we don't enable HugeTLB if the cache is not configured.
I am still not sure about this. IMHO hugetlb support should be a hardware
support derivative and any cache allocation failure should be handled as I did
in the earlier patch. But then if we were not able to create hugetlb page table
cache, we can as well declare hugetlb support disabled thereby avoiding calling
into allocation routines.

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