wgengine/magicsock: remove TODO

The TODO above derphttp.NewClient suggests it does network I/O,
but the derphttp client connects lazily and so creating one is
very cheap.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -706,7 +706,8 @@ func (c *Conn) derpWriteChanOfAddr(addr *net.UDPAddr) chan<- derpWriteRequest {
 			return nil
 		}
 
-		// TODO(bradfitz): don't hold derpMu here. It's slow. Release first and use singleflight to dial+re-lock to add.
+		// Note that derphttp.NewClient does not dial the server
+		// so it is safe to do under the derpMu lock.
 		dc, err := derphttp.NewClient(c.privateKey, "https://"+derpSrv.HostHTTPS+"/derp", c.logf)
 		if err != nil {
 			c.logf("derphttp.NewClient: port %d, host %q invalid? err: %v", addr.Port, derpSrv.HostHTTPS, err)