wgengine: use set.HandleSet in another place

I guess we missed this one earlier when we unified the various
copies into set.HandleSet.

Updates #cleanup

Change-Id: I7e6de9ce16e8fc4846abf384dfcc8eaec4d99e60
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/userspace.go

@@ -133,7 +133,7 @@ type userspaceEngine struct {
 	peerSequence        []key.NodePublic
 	endpoints           []tailcfg.Endpoint
 	pendOpen            map[flowtrack.Tuple]*pendingOpenFlow // see pendopen.go
-	networkMapCallbacks map[*someHandle]NetworkMapCallback
+	networkMapCallbacks set.HandleSet[NetworkMapCallback]
 	tsIPByIPPort        map[netip.AddrPort]netip.Addr // allows registration of IP:ports as belonging to a certain Tailscale IP for whois lookups
 
 	// pongCallback is the map of response handlers waiting for disco or TSMP
@@ -1162,10 +1162,9 @@ func (e *userspaceEngine) AddNetworkMapCallback(cb NetworkMapCallback) func() {
 	e.mu.Lock()
 	defer e.mu.Unlock()
 	if e.networkMapCallbacks == nil {
-		e.networkMapCallbacks = make(map[*someHandle]NetworkMapCallback)
+		e.networkMapCallbacks = make(set.HandleSet[NetworkMapCallback])
 	}
-	h := new(someHandle)
-	e.networkMapCallbacks[h] = cb
+	h := e.networkMapCallbacks.Add(cb)
 	return func() {
 		e.mu.Lock()
 		defer e.mu.Unlock()

wgengine/wgengine.go

@@ -42,11 +42,6 @@ type Status struct {
 // into network map updates.
 type NetworkMapCallback func(*netmap.NetworkMap)
 
-// someHandle is allocated so its pointer address acts as a unique
-// map key handle. (It needs to have non-zero size for Go to guarantee
-// the pointer is unique.)
-type someHandle struct{ _ byte }
-
 // ErrNoChanges is returned by Engine.Reconfig if no changes were made.
 var ErrNoChanges = errors.New("no changes made to Engine config")