Fix leak in netstack's tunnel

The upstream netstack's virtual tunnel device has a leaky Close
implementation - the virtual network is not being shut down properly.
What's missing is this one line change to shut down all the go routines
associated with the userspace networking stack. This change is verbatim
copied from kshangx's PR - WireGuard/wireguard-go#101

Signed-off-by: kshangx <[email protected]>

Author: kshangx

tun/netstack/tun.go

@@ -157,6 +157,7 @@ func (tun *netTun) Flush() error {
 
 func (tun *netTun) Close() error {
 	tun.stack.RemoveNIC(1)
+	tun.stack.Close()
 
 	if tun.events != nil {
 		close(tun.events)