selftests/powerpc: Fix core-pkey for default execute permission change

Only when the key is allocated, its permission are enabled.

Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>