Don't collect metrics from overlay filesystems

This is typically used to mount Docker containers but it generates too
many volatile metrics which aren't useful.

Change-Id: I00117895570515b2c8f9690542e83061309464c3
diff --git a/linux/meta/collectd.yml b/linux/meta/collectd.yml
index 0bd2b55..d38f1ae 100644
--- a/linux/meta/collectd.yml
+++ b/linux/meta/collectd.yml
@@ -36,6 +36,7 @@
     - tmpfs
     - fusectl
     - cgroup
+    - overlay
   linux_storage_disk:
     plugin: disk
     template: linux/files/collectd_disk.conf