This is to address a regression introduced in #131118.
When syncing the first dataset, syncoid expects that the target
dataset doesn't exist to have a clean slate to work with. So during
runtime we'll check if the target dataset does exist and if it doesn't
- delegate the permissions to the parent dataset instead.
But then, on unallow, we do the unallow on both the target and the
parent since the target dataset should have been created at this
point, so the unallow can't know which dataset that got permissions
just by which datasets exists.
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