dump: save master_id for the root mount
dump: save master_id for the root mount

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-43260
Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

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