spapr: Fix stale HTAB during live migration (TCG)

If a TCG guest reboots during a running migration HTAB entries are not
marked dirty, and the destination boots with an invalid HTAB.

When a reboot occurs, explicitly mark the current HTAB dirty after
clearing it.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>