net/portmapper: Stop replacing the internal port with the upnp external port (#18349)

net/portmapper: Stop replacing the internal port with the upnp external port

This causes the UPnP mapping to break in the next recreation of the
mapping.

Fixes #18348

Signed-off-by: Eduardo Sorribas <eduardo@sorribas.org>

net/portmapper/upnp.go

@@ -574,7 +574,7 @@ type step struct {
 		c.mu.Lock()
 		defer c.mu.Unlock()
 		c.mapping = upnp
-		c.localPort = externalAddrPort.Port()
+		c.localPort = internal.Port()
 		return upnp.external, true
 	}