powerpc/powernv: Avoid crashing if rng is NULL

powerpc/powernv: Avoid crashing if rng is NULL

On a bare-metal Power8 system that doesn't have an "ibm,power-rng", a
malicious QEMU and guest that ignore the absence of the
KVM_CAP_PPC_HWRNG flag, and calls H_RANDOM anyway, will dereference a
NULL pointer.

In practice all Power8 machines have an "ibm,power-rng", but let's not
rely on that, add a NULL check and early return in
powernv_get_random_real_mode().

Fixes: 1746d4c9c4c7 ("KVM: PPC: Book3S HV: Add fast real-mode H_RANDOM implementation.")
Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220727143219.2684192-1-mpe@ellerman.id.au