Memory (FreeBSD): fixing disk cache fetching

[crocidb]

Summary

Memory fetching for FreeBSD was using vm.stats.vm.v_cache_count as cache, but FreeBSD's kernel doesn't have cache pages since version 12. The value is still there for compatibility, but it returns always 0.

Changes

• using filesystem buffer cache and ARC cache: vfs.bufspace tracks the bytes used to buffer the filesystem addressing for other filesystems than ZFS, and ARC tracks buffers AND cache for ZFS, which bypasses bufspace entirely. I also decrease the minimum ARC cache, since that's arguably non retrievable memory. The results now are more aligned with how other monitoring tools track memory. In fact, I submitted PRs for btop and htop to fix similar issues.

Checklist

• I have tested my changes locally

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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