nat: do not shortcircuit permanent mappings

If we use same NAT agent and call for the same permanent mapping
again we get the same mapping, no harm done.

If router dies, we will remap again.
Just pros, no cons.

nat.go

@@ -163,6 +163,8 @@ type mapping struct {
 	intaddr   ma.Multiaddr
 	proc      goprocess.Process
 
+	comment string
+
 	cached    ma.Multiaddr
 	cacheTime time.Time
 	cacheLk   sync.Mutex
@@ -198,18 +200,6 @@ func (m *mapping) setExternalPort(p int) {
 	m.extport = p
 }
 
-func (m *mapping) setPermanent(p bool) {
-	m.Lock()
-	defer m.Unlock()
-	m.permanent = p
-}
-
-func (m *mapping) isPermanent() bool {
-	m.Lock()
-	defer m.Unlock()
-	return m.permanent
-}
-
 func (m *mapping) InternalAddr() ma.Multiaddr {
 	m.Lock()
 	defer m.Unlock()
@@ -346,20 +336,17 @@ func (nat *NAT) NewMapping(maddr ma.Multiaddr) (Mapping, error) {
 
 func (nat *NAT) establishMapping(m *mapping) {
 	oldport := m.ExternalPort()
-	if oldport != 0 && m.isPermanent() {
-		// mapping was already established and it is permanent
-		return
-	}
 
 	log.Debugf("Attempting port map: %s/%d", m.Protocol(), m.InternalPort())
-	permanent := false
-
-	newport, err := nat.nat.AddPortMapping(m.Protocol(), m.InternalPort(), "libp2p", MappingDuration)
+	comment := "libp2p"
+	if m.comment != "" {
+		comment = "libp2p-" + m.comment
+	}
 
+	newport, err := nat.nat.AddPortMapping(m.Protocol(), m.InternalPort(), comment, MappingDuration)
 	if err != nil {
 		// Some hardware does not support mappings with timeout, so try that
-		newport, err = nat.nat.AddPortMapping(m.Protocol(), m.InternalPort(), "libp2p", 0)
-		permanent = (err == nil)
+		newport, err = nat.nat.AddPortMapping(m.Protocol(), m.InternalPort(), comment, 0)
 	}
 
 	failure := func() {
@@ -379,7 +366,6 @@ func (nat *NAT) establishMapping(m *mapping) {
 		return
 	}
 
-	m.setPermanent(permanent)
 	m.setExternalPort(newport)
 	ext, err := m.ExternalAddr()
 	if err != nil {