cmd/tailscaled,net/tstun: fix data race on start-up in TUN mode

Fixes #7894

Change-Id: Ice3f8019405714dd69d02bc07694f3872bb598b8

Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>

tsnet/tsnet.go

@@ -530,6 +530,7 @@ func (s *Server) start() (reterr error) {
 	if err != nil {
 		return fmt.Errorf("netstack.Create: %w", err)
 	}
+	sys.Tun.Get().Start()
 	sys.Set(ns)
 	ns.ProcessLocalIPs = true
 	ns.GetTCPHandlerForFlow = s.getTCPHandlerForFlow