Jobs disk usage doesn't work when the workspace directory is a symbolic link on Linux filesystem

### Jenkins and plugins versions report

Environment

```text
Jenkins 2.573
CloudBees Disk Usage Simple 275.v56e74c9dd0d3
Safari 26.5.2
```

### What Operating System are you using (both controller, and any agents involved in the problem)?

Red Hat Enterprise Linux release 9.8 (Plow)

### Reproduction steps

1. Make sure your "workspace" directory in Jenkins home is a symbolic link to another directory or another disk;
2. Go to the Disk Usage page and try to get results.

```Bash
[root@abc jenkins]# ls -la workspace
lrwxrwxrwx. 1 root root 31 Aug 7 2025 workspace -> /data/var/lib/jenkins/workspace
```

### Expected Results

Get disk usage results.

### Actual Results

Get nothing.

### Anything else?

_No response_

### Are you interested in contributing a fix?

_No response_

Start by reproducing the issue on the Jenkins Disk Usage page with the Jenkins workspace directory configured as a symbolic link on a Linux filesystem. Trace the disk-usage calculation from that page and verify that it returns usage results for the linked workspace instead of nothing.

Written by the indexing model from the issue text.