MFC r248493:
Plug memory leak in dsl_check_snap_cb()
This was unnoticed because the function is very rarely used.
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:

	# zfs destroy -t tank@snap

eventhough tank@snap doesn't exit.

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

	# 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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