eth: fix shutdown regression to abort downloads, not just cancel

eth/downloader/downloader.go

@@ -557,6 +557,8 @@ func (d *Downloader) spawnSync(fetchers []func() error) error {
 func (d *Downloader) cancel() {
 	// Close the current cancel channel
 	d.cancelLock.Lock()
+	defer d.cancelLock.Unlock()
+
 	if d.cancelCh != nil {
 		select {
 		case <-d.cancelCh:
@@ -565,7 +567,6 @@ func (d *Downloader) cancel() {
 			close(d.cancelCh)
 		}
 	}
-	d.cancelLock.Unlock()
 }
 
 // Cancel aborts all of the operations and waits for all download goroutines to