The root mount is always external and master_id is external too.
It's required to validate mounts. We already do this on restore,
but we need to do this on dump too. Otherwise we will get the error:

Mount %d %s (master_id: %d shared_id: %d) has unreachable sharing. Try --enable-external-masters.

https://jira.sw.ru/browse/PSBM-43260Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>