selftests/powerpc: ptrace-pkey: Update the test to mark an invalid pkey correctly

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/20200709032946.881753-22-aneesh.kumar@linux.ibm.com