Fix double-counted Linux disk space from bind-mounted filesystems (#44969)

**Related issue:** Resolves #43091

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

- [x] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements), JS
inline code is prevented especially for url redirects, and untrusted
data interpolated into shell scripts/commands is validated against shell
metacharacters.

## Testing

- [ ] Added/updated automated tests

- [x] QA'd all new/changed functionality manually

Was able to reproduce by enrolling a Ubuntu 25.10 host and mounting /
onto a different path like this:

```bash
sudo mkdir -p /tmp/snap.rootfs_TESTING
sudo mount --bind / /tmp/snap.rootfs_TESTING
```

Then, refetched vitals and saw the total disk space doubled:
<img width="211" height="108" alt="Screenshot 2026-05-07 at 9 40 12 PM"
src="https://github.com/user-attachments/assets/81cad4af-f146-4ea9-84e3-ee56eb426d8a"
/>

With the fix applied on this branch, refetched again and saw the correct
total disk space:

<img width="251" height="118" alt="Screenshot 2026-05-07 at 9 31 15 PM"
src="https://github.com/user-attachments/assets/cbb6f91b-320a-4d48-8583-f5fdf5a4fcd9"
/>

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Bug Fixes
- Resolved inaccurate total disk space reporting on Linux systems where
the same filesystem is bind-mounted at multiple paths.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

changes/43091-inaccurate-total-disk-space

@@ -0,0 +1 @@
+* Fixed Linux total disk space being double-counted when a filesystem was bind-mounted at multiple paths (e.g. snap-confine's `/tmp/snap.rootfs_*`).