The performance issue in #19467 drove me to add a benchmark for
system-df to avoid regressing on it in the future.
Comparing current HEAD to v4.6.0 yields
```
/home/vrothberg/containers/podman/bin/podman system df ran
201.47 times faster than /usr/bin/podman system df
```
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>
