zfs: fix dataset name parsing

[vigneshakaviki]

Summary

• Parse the ZFS objset dataset value after the kstat type column instead of searching the full line for the value.
• Preserve repeated spaces inside valid dataset names.
• Add regression coverage for a root dataset whose name also occurs in dataset_name.

This prevents root pool names such as data from being exported as dataset_name 7 data.

Testing

• make in Linux Go 1.26 container
• go test -race ./...
• GOARCH=amd64 go test ./collector -run '^TestZpoolObjsetParsing' -count=1
• GOARCH=386 go test -c ./collector
• govulncheck ./...

Fixes #3761

@SuperQ, please review.