Process monitor: Ignore missing /proc/net/tcp6

This makes Packetbeat's process monitor to continue execution when
/proc/net/tcp6 is missing (ipv6 disabled in kernel).

Closes elastic#19941

CHANGELOG.next.asciidoc

@@ -286,6 +286,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 *Packetbeat*
 
 - Enable setting promiscuous mode automatically. {pull}11366[11366]
+- Fix process monitoring when ipv6 is disabled under Linux. {issue}19941[19941] {pull}19945[19945]
 
 *Winlogbeat*
 

packetbeat/procs/procs_linux.go

@@ -68,8 +68,10 @@ func (proc *ProcessesWatcher) GetLocalPortToPIDMapping(transport applayer.Transp
 		logp.Err("GetLocalPortToPIDMapping: parsing '%s': %s", sourceFiles.ipv4, err)
 		return nil, err
 	}
+
 	ipv6socks, err := socketsFromProc(sourceFiles.ipv6, true)
-	if err != nil {
+	// Ignore the error when /proc/net/tcp6 doesn't exists (ipv6 disabled).
+	if err != nil && !os.IsNotExist(err) {
 		logp.Err("GetLocalPortToPIDMapping: parsing '%s': %s", sourceFiles.ipv6, err)
 		return nil, err
 	}