all: use ++ to increment

Make the code slightly more idiomatic. No functional changes.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/device.go

@@ -323,7 +323,7 @@ func NewDevice(tunDevice tun.Device, logger *Logger) *Device {
 
 	cpus := runtime.NumCPU()
 	device.state.stopping.Wait()
-	for i := 0; i < cpus; i += 1 {
+	for i := 0; i < cpus; i++ {
 		device.state.stopping.Add(2) // decryption and handshake
 		go device.RoutineEncryption()
 		go device.RoutineDecryption()