ZTS: parameter expansion in zfs_unshare_006_pos

zfs_unshare_006 checks to see if a dataset still has an active SMB
share after doing an NFS unshare -a. The test could fail because the
check for the SMB share does not expect dashes in a dataset name to be
converted to underscores as pathname delimiters are.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #13893