control/controlclient: avoid calls to ms.netmap() (#19281)

Instead of generating the full netmap, just fetch the peers out the the
existing peers map.

The extra usage was introduced with netmap caching, but there is no need
to call the netmap to get this information, rather the existing peermap
can be used.

Updates #12639

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

control/controlclient/map_test.go

@@ -696,13 +696,12 @@ func TestUpdateDiscoForNode(t *testing.T) {
 			ms.updateDiscoForNode(node.ID, node.Key, newKey.Public(), tt.updateLastSeen, tt.updateOnline)
 			<-nu.done
 
-			nm := ms.netmap()
-			peerIdx := nm.PeerIndexByNodeID(node.ID)
-			if peerIdx == -1 {
+			peer, ok := ms.peers[node.ID]
+			if !ok {
 				t.Fatal("node not found")
 			}
 
-			updated := nm.Peers[peerIdx].DiscoKey().Compare(newKey.Public()) == 0
+			updated := peer.DiscoKey().Compare(newKey.Public()) == 0
 			if updated != tt.wantUpdate {
 				t.Fatalf("Disco key update: %t, wanted update: %t", updated, tt.wantUpdate)
 			}