wgengine/netstack: do not send packets to netstack after close

Use the local context on Impl to check for shut down state in order to
drop rather than inject packets after close has begun.

Netstack sets endpoint.dispatcher to nil during shutdown. After the
recent adjustment in 920ec69241 we now
wait for netstack to fully shutdown before we release tests. This means
that we may continue to accept packets and attempt to inject them, which
we must prevent in order to avoid nil pointer panic.

References google/gvisor#8765
Fixes #7715

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/netstack/netstack.go

@@ -394,6 +394,10 @@ func (ns *Impl) updateIPs(nm *netmap.NetworkMap) {
 // the host and arriving at tailscaled. This method returns filter.DropSilently
 // to intercept a packet for handling, for instance traffic to quad-100.
 func (ns *Impl) handleLocalPackets(p *packet.Parsed, t *tstun.Wrapper) filter.Response {
+	if ns.ctx.Err() != nil {
+		return filter.DropSilently
+	}
+
 	// If it's not traffic to the service IP (i.e. magicDNS) we don't
 	// care; resume processing.
 	if dst := p.Dst.Addr(); dst != magicDNSIP && dst != magicDNSIPv6 {
@@ -670,6 +674,10 @@ func (ns *Impl) userPing(dstIP netip.Addr, pingResPkt []byte) {
 // whereas returning filter.DropSilently is done when netstack intercepts the
 // packet and no further processing towards to host should be done.
 func (ns *Impl) injectInbound(p *packet.Parsed, t *tstun.Wrapper) filter.Response {
+	if ns.ctx.Err() != nil {
+		return filter.DropSilently
+	}
+
 	if !ns.shouldProcessInbound(p, t) {
 		// Let the host network stack (if any) deal with it.
 		return filter.Accept