powerpc/mm/hugetlb: Don't panic when we don't find the default huge page size

The generic hugetlbfs code can handle not finding the default huge page
size correctly. With HPAGE_SHIFT = 0 we see in dmesg:

  hugetlbfs: disabling because there are no supported hugepage sizes

bash-4.2# echo 30 > /proc/sys/vm/nr_hugepages
bash: echo: write error: Operation not supported

Fixes: 03bb2d65 ("powerpc: get hugetlbpage handling more generic")
Reported-by: Chris Smart <chris@distroguy.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>