zfs userspace: use zfs_path_to_zhandle so argument can be a path

Change zfs userspace subcommand to use zfs_path_to_zhandle() so that
the provided dataset can be a path (/usr) or a dataset (rpool/usr).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #8915