selftests/powerpc: ptrace-pkey: Rename variables to make it easier to follow code

Rename variable to indicate that they are invalid values which we will
use to test ptrace update of pkeys.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200709032946.881753-21-aneesh.kumar@linux.ibm.com