powerpc/rtas: block error injection when locked down

The error injection facility on pseries VMs allows corruption of
arbitrary guest memory, potentially enabling a sufficiently privileged
user to disable lockdown or perform other modifications of the running
kernel via the rtas syscall.

Block the PAPR error injection facility from being opened or called
when locked down.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Acked-by: Paul Moore <paul@paul-moore.com> (LSM)
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220926131643.146502-3-nathanl@linux.ibm.com