compel/ppc64le: make task size detection more robust

The recent changes to user address space limits in the Linux kernel break
the assumption that TASK_SIZE is 128TB. For now, the maximal task size on
ppc64le is 512TB and we need to detect it in runtime for compatibility with
older kernels.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>