FreeBSD: possible compilation issues due to zfs collector.

The changes introduced in this [PR](https://github.com/prometheus/node_exporter/pull/2753) likely will make node exporter not compile for freebsd:

This [line](https://github.com/prometheus/node_exporter/blob/master/collector/zfs_freebsd.go#L339C4-L339C19) doesnt match the initialized variable `zfsDatasetsNames`.

