tun/netstack: fix race during (*netTun).Close()

[134130]

What

• Use ep.ReadContext to detect the netTun is closed, and remove the WriteNotify implementation.
• This is more simpler than owning some channel
• I've tested this in my internal project

Why

• I also experienced the same problem as tun/netstack: fix race during (*netTun).Close() #85
• When closing the netTun while the (*endpoint) is on handshaking, the Close() blocks forever, because the incomingPacket channel is closed but Write() trying to send a view.
• Therefore, when using netTun with http.Transport, you will encounter a phenomenon where the number of goroutines gradually increases over time.

[134130]

need to add view.Release