It's possible for a Hypervisor Maintenance Interrupt (HMI) to occur while in
the pmap code, holding locks. This can cause WITNESS to panic due to lock
errors in calling pmap_kextract(). Since we don't yet handle the flags
returned by OPAL_HANDLE_HMI2, just stop using it, so that we don't call into
pmap_kextract().
Reported by: pkubaj