KVM: PPC: Book3S HV: Don't drop low-order page address bits

Commit caaa4c80 ("KVM: PPC: Book3S HV: Fix physical address
calculations") unfortunately resulted in some low-order address bits
getting dropped in the case where the guest is creating a 4k HPTE
and the host page size is 64k.  By getting the low-order bits from
hva rather than gpa we miss out on bits 12 - 15 in this case, since
hva is at page granularity.  This puts the missing bits back in.

Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>