sysutils/munin-node: Fix plugins/zfs_compress if label contains . (dot)

Fixes "[ERROR] config error" that occurs if "." (dot) is in label

PR: 281118
Reported by: takefu <takefu@airport.fm>
Tested by: John W. O'Brien <john@saltant.com>
Approved by: portmgr (maintainer timeout, 3+ months)