KVM: PPC: Book3S HV: Disable HPT resizing on POWER9 for now

The new HPT resizing code added in commit b5baa687

 ("KVM: PPC:
Book3S HV: KVM-HV HPT resizing implementation", 2016-12-20) doesn't
have code to handle the new HPTE format which POWER9 uses.  Thus it
would be best not to advertise it to userspace on POWER9 systems
until it works properly.

Also, since resize_hpt_rehash_hpte() contains BUG_ON() calls that
could be hit on POWER9, let's prevent it from being called on POWER9
for now.

Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>