Add overlay to defIgnoredFSTypes (#824)

* Add overlay to defIgnoredFSTypes

To avoid statfs() errors if node_exporter is running as non privileged user.

* Updated defIngoredFSTypes values in sorted order

collector/filesystem_linux.go

@@ -26,7 +26,7 @@ import (
 
 const (
 	defIgnoredMountPoints = "^/(dev|proc|sys|var/lib/docker)($|/)"
-	defIgnoredFSTypes     = "^(autofs|binfmt_misc|cgroup|configfs|debugfs|devpts|devtmpfs|fusectl|hugetlbfs|mqueue|proc|procfs|pstore|rpc_pipefs|securityfs|sysfs|tracefs)$"
+	defIgnoredFSTypes     = "^(autofs|binfmt_misc|cgroup|configfs|debugfs|devpts|devtmpfs|fusectl|hugetlbfs|mqueue|overlay|proc|procfs|pstore|rpc_pipefs|securityfs|sysfs|tracefs)$"
 	readOnly              = 0x1 // ST_RDONLY
 )