net/dns: don't timeout if inotify sends multiple events

This fixes a flaky test which has been occasionally timing out in CI.

In particular, this test times out if `watchFile` receives multiple
notifications from inotify before we cancel the test context. We block
processing the second notification, because we've stopped listening to
the `callbackDone` channel.

This patch changes the test so we only send on the first notification.

Testing this locally with `stress` confirms that the test is no longer
flaky.

Fixes #17172
Updates #14699

Signed-off-by: Alex Chan <alexc@tailscale.com>

net/dns/direct_linux_test.go

@@ -25,8 +25,13 @@ func TestWatchFile(t *testing.T) {
 	var callbackCalled atomic.Bool
 	callbackDone := make(chan bool)
 	callback := func() {
-		callbackDone <- true
-		callbackCalled.Store(true)
+		// We only send to the channel once to avoid blocking if the
+		// callback is called multiple times -- this happens occasionally
+		// if inotify sends multiple events before we cancel the context.
+		if !callbackCalled.Load() {
+			callbackDone <- true
+			callbackCalled.Store(true)
+		}
 	}
 
 	var eg errgroup.Group