control/controlclient: avert a data race when logging (#8863)

The read of the synced field for logging takes place outside the lock, and
races with other (locked) writes of this field, including for example the one
at current line 556 in mapRoutine.

Updates tailscale/corp#13856

Change-Id: I056b36d7a93025aafdf73528dd7645f10b791af6
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

control/controlclient/auto.go

@@ -313,9 +313,12 @@ func (c *Auto) cancelMap() {
 // restartMap cancels the existing mapPoll and liteUpdates, and then starts a
 // new one.
 func (c *Auto) restartMap() {
-	c.cancelMap()
+	c.mu.Lock()
+	c.cancelMapLocked()
+	synced := c.synced
+	c.mu.Unlock()
 
-	c.logf("[v1] restartMap: synced=%v", c.synced)
+	c.logf("[v1] restartMap: synced=%v", synced)
 
 	select {
 	case c.newMapCh <- struct{}{}: