azureblob,oracleobjectstorage,s3: quit multipart uploads if the context is cancelled

Before this change the multipart uploads would continue retrying even
if the context was cancelled.

backend/azureblob/azureblob.go

@@ -2162,6 +2162,9 @@ func (w *azChunkWriter) WriteChunk(ctx context.Context, chunkNumber int, reader
 			if chunkNumber <= 8 {
 				return w.f.shouldRetry(ctx, err)
 			}
+			if fserrors.ContextError(ctx, &err) {
+				return false, err
+			}
 			// retry all chunks once have done the first few
 			return true, err
 		}