wgengine/magicsock: quiet log flood at tailscaled shutdown

When you hit control-C on a tailscaled (notably in dev mode, but
also on any systemctl stop/restart), there is a flood of messages like:

magicsock: doing cleanup for discovery key d:aa9c92321db0807f
magicsock: doing cleanup for discovery key d:bb0f16aacadbfd46
magicsock: doing cleanup for discovery key d:b5b2d386296536f2
magicsock: doing cleanup for discovery key d:3b640649f6796c91
magicsock: doing cleanup for discovery key d:71d7b1afbcce52cd
magicsock: doing cleanup for discovery key d:315b61d7e0111377
magicsock: doing cleanup for discovery key d:9301f63dce69bf45
magicsock: doing cleanup for discovery key d:376141884d6fe072
....

It can be hundreds or even tens of thousands.

So don't do that. Not a useful log message during shutdown.

Change-Id: I029a8510741023f740877df28adff778246c18e5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -353,7 +353,8 @@ type Conn struct {
 	mu     sync.Mutex
 	muCond *sync.Cond
 
-	closed bool // Close was called
+	closed  bool        // Close was called
+	closing atomic.Bool // Close is in progress (or done)
 
 	// derpCleanupTimer is the timer that fires to occasionally clean
 	// up idle DERP connections. It's only used when there is a non-home
@@ -2859,6 +2860,7 @@ func (c *Conn) Close() error {
 	if c.closed {
 		return nil
 	}
+	c.closing.Store(true)
 	if c.derpCleanupTimerArmed {
 		c.derpCleanupTimer.Stop()
 	}
@@ -4441,7 +4443,9 @@ func (de *endpoint) stopAndReset() {
 	de.mu.Lock()
 	defer de.mu.Unlock()
 
-	de.c.logf("[v1] magicsock: doing cleanup for discovery key %s", de.discoKey.ShortString())
+	if closing := de.c.closing.Load(); !closing {
+		de.c.logf("[v1] magicsock: doing cleanup for discovery key %s", de.discoKey.ShortString())
+	}
 
 	de.resetLocked()
 	if de.heartBeatTimer != nil {