Fix issue when destroying snapshot of top-level dataset

To extract the pool name from a snapshot name, a parameter expansion was removing everything after '/'. However, the top-level dataset contains no '/' in its name (its name is the same as the pool's name). This means the entire snapshot name was being incorrectly returned for the name of the pool.

This release fixes the parameter expansion to handle the case when the dataset name contains no '/'.

This issue was reported by Michael Wayne.