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

[ Upstream commit 9a11f12e0a6c374b3ef1ce81e32ce477d28eb1b8 ]

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


Signed-off-by: Sasha Levin <sashal@kernel.org>