powerpc/mm: Handle page table allocation failures

This fixes kernel crash that arises due to not handling page table allocation
failures while allocating hugetlb page table.

Fixes: e2b3d202 ("powerpc: Switch 16GB and 16MB explicit hugepages to a different page table format")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>