net/memnet: allow listener address reuse (#17342)

Listen address reuse is allowed as soon as the previous listener is
closed. There is no attempt made to emulate more complex address reuse
logic.

Updates tailscale/corp#28078

Change-Id: I56be1c4848e7b3f9fc97fd4ef13a2de9dcfab0f2

Signed-off-by: Brian Palmer <brianp@tailscale.com>

net/memnet/memnet.go

@@ -61,6 +61,11 @@ func (m *Network) Listen(network, address string) (net.Listener, error) {
 		}
 		ln := Listen(key)
 		m.lns[key] = ln
+		ln.onClose = func() {
+			m.mu.Lock()
+			delete(m.lns, key)
+			m.mu.Unlock()
+		}
 		return ln, nil
 	}
 }