selftests/mm: add support to test 4PB VA on PPC64

selftests/mm: add support to test 4PB VA on PPC64

PowerPC64 supports a 4PB virtual address space, but this test was
previously limited to 512TB.  This patch extends the coverage up to the
full 4PB VA range on PowerPC64.

Memory from 0 to 128TB is allocated without an address hint, while
allocations from 128TB to 4PB use a hint address.

Link: https://lkml.kernel.org/r/20250816040113.760010-3-aboorvad@linux.ibm.com
Co-developed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>