x86: machine_restart() must not call acpi_dmar_reinstate() twice

x86: machine_restart() must not call acpi_dmar_reinstate() twice

.. as that function is not idempotent (it always alters the table
checksum). The (generally) duplicate call was a result from it being
made before machine_restart() re-invoking itself on the boot CPU.

Considering that no problem arose so far from the table corruption I
doubt that we need to restore the correct table signature on the
reboot path in general. The only case I can see this as potentially
necessary is the tboot one, hence do the call just in that case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>