gcs: fix a deadlock in gs parallel writer (pingcap#49800)

ref pingcap#48443

Author: lance6716

br/pkg/storage/gcs_extra.go

@@ -144,7 +144,7 @@ func (w *GCSWriter) readChunk(ch chan chunk) {
 		select {
 		case <-w.ctx.Done():
 			data.cleanup()
-			return
+			w.err.CompareAndSwap(nil, w.ctx.Err())
 		default:
 			part := &xmlMPUPart{
 				uploadBase: w.uploadBase,