KVM: PPC: Book3S HV: Make radix clear pte when unmapping

The current partition table unmap code clears the _PAGE_PRESENT bit
out of the pte, which leaves pud_huge/pmd_huge true and does not
clear pud_present/pmd_present.  This can confuse subsequent page
faults and possibly lead to the guest looping doing continual
hypervisor page faults.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>