This disables the use of the streamlined entry path for radix guests
on early POWER9 chips that need the workaround added in commit
a25bd72b ("powerpc/mm/radix: Workaround prefetch issue with KVM",
2017-07-24), because the streamlined entry path does not include
that workaround.  This also means that we can't do nested HV-KVM
on those chips.

Since the chips that need that workaround are the same ones that can't
run both radix and HPT guests at the same time on different threads of
a core, we use the existing 'no_mixing_hpt_and_radix' variable that
identifies those chips to identify when we can't use the new guest
entry path, and when we can't do nested virtualization.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>