Fix ephemeral port leak.

Fix a bug where udp.(*endpoint).Disconnect [accessible in gVisor via
epsocket.(*SocketOperations).Connect with AF_UNSPEC] would leak a port
reservation if the socket/endpoint had an ephemeral port assigned to it.

glibc's getaddrinfo uses connect with AF_UNSPEC, causing each call of
getaddrinfo to leak a port. Call getaddrinfo too many times and you run out of
ports (shows up as connect returning EAGAIN and getaddrinfo returning
EAI_NONAME "Name or service not known").

PiperOrigin-RevId: 268071160

pkg/tcpip/transport/udp/endpoint.go

@@ -747,6 +747,10 @@ func (e *endpoint) Disconnect() *tcpip.Error {
 		}
 		e.state = StateBound
 	} else {
+		if e.id.LocalPort != 0 {
+			// Release the ephemeral port.
+			e.stack.ReleasePort(e.effectiveNetProtos, ProtocolNumber, e.id.LocalAddress, e.id.LocalPort)
+		}
 		e.state = StateInitial
 	}