Remove error log when snmp6 directory does not exists with nstat input (#5403) (#5413)

plugins/inputs/nstat/README.md

@@ -36,6 +36,8 @@ The sample config file
   # dump_zeros			= 	true
 ```
 
+In case that `proc_net_snmp6` path doesn't exist (e.g. IPv6 is not enabled) no error would be raised.
+
 ### Measurements & Fields
 
 - nstat

plugins/inputs/nstat/nstat.go

@@ -83,13 +83,14 @@ func (ns *Nstat) Gather(acc telegraf.Accumulator) error {
 		return err
 	}
 
-	// collect SNMP6 data
+	// collect SNMP6 data, if SNMP6 directory exists (IPv6 enabled)
 	snmp6, err := ioutil.ReadFile(ns.ProcNetSNMP6)
+	if err == nil {
+		err = ns.gatherSNMP6(snmp6, acc)
 		if err != nil {
 			return err
 		}
-	err = ns.gatherSNMP6(snmp6, acc)
-	if err != nil {
+	} else if !os.IsNotExist(err) {
 		return err
 	}
 	return nil