collectd: df: Ignore autofs mount points

When _collectd_ calls *statvfs(3)* on paths like
`/host/proc/sys/fs/binfmt_misc` which are configured for auto-mounting,
_systemd_ logs hundreds of messages like these:

```
systemd[1]: proc-sys-fs-binfmt_misc.automount: Got automount request for /proc/sys/fs/binfmt_misc, triggered by 1303 (reader#3)
systemd[1]: proc-sys-fs-binfmt_misc.automount: Automount point already active?
```

Eventually, _collectd_ logs an error:

```
collectd[1132]: statvfs(/host/proc/sys/fs/binfmt_misc) failed: Too many levels of symbolic links
```

This happens on every scrape interval.

To avoid this, we can configure _collectd_ to skip calling *statvfs(3)*
on _autofs_ mount points.  Even if it did work correctly, we wouldn't
really want _collectd_ triggering automounts; that would pretty much
defeat the purpose of them.

templates/collectd/collectd-df.conf

@@ -3,6 +3,7 @@ LoadPlugin df
 <Plugin df>
     ReportByDevice true
 
+    FSType autofs
     FSType overlay
     IgnoreSelected true
 </Plugin>