powerpc/64: Disable use of radix under a hypervisor

Currently, if the kernel is running on a POWER9 processor under a
hypervisor, it may try to use the radix MMU even though it doesn't have
the necessary code to do so (it doesn't negotiate use of radix, and it
doesn't do the H_REGISTER_PROC_TBL hcall).  If the hypervisor supports
both radix and HPT, then it will set up the guest to use HPT (since the
guest doesn't request radix in the CAS call), but if the radix feature
bit is set in the ibm,pa-features property (which is valid, since
ibm,pa-features is defined to represent the capabilities of the
processor) the guest will try to use radix, resulting in a crash when
it turns the MMU on.

This makes the minimal fix for the current code, which is to disable
radix unless we are running in hypervisor mode.

Fixes: 2bfd65e4 ("powerpc/mm/radix: Add radix callbacks for early init routines")
Cc: stable@vger.kernel.org # v4.7+
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>