MFC r240870 (pjd):
It is possible to recursively destroy snapshots even if the snapshot
doesn't exist on a dataset we are starting from. For example if we
have the following configuration:

tank
tank/foo
tank/foo@snap
tank/bar
tank/bar@snap

We can execute:

1. zfs destroy -t tank@snap

eventhough tank@snap doesn't exit.

Unfortunately it is not possible to do the same with recursive rename:

1. zfs rename -r tank@snap tank@pans

cannot open 'tank@snap': dataset does not exist

...until now. This change allows to recursively rename snapshots even if
snapshot doesn't exist on the starting dataset.

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