quatrix: fix chunk upload retries and fix memory leak

Each upload chunk is buffered in a pool.RW from the global memory pool
but was never closed, so its pages were never returned to the pool.

Close the buffer after each chunk is uploaded and on the read error
path.

A chunk that failed with a retryable error was also retried without
rewinding the buffer, so the retry sent an empty body with the original
Content-Length and Content-Range and failed.

Seek the chunk back to the start inside the pacer closure so each
attempt re-sends it in full.

The FsPutRetry integration test covers the retry of a failed upload
request and checks the buffers are returned to the pool.

backend/quatrix/quatrix.go

@@ -1126,12 +1126,14 @@ func (o *Object) dynamicUpload(ctx context.Context, size int64, modTime time.Tim
 
 		_, err := io.CopyN(rw, in, localChunk)
 		if err != nil {
+			_ = rw.Close()
 			return fmt.Errorf("read chunk with offset %d size %d: %w", offset, localChunk, err)
 		}
 
 		start := time.Now()
 
 		err = o.upload(ctx, uploadSession.UploadKey, rw, size, offset, localChunk, options...)
+		_ = rw.Close()
 		if err != nil {
 			return fmt.Errorf("upload chunk with offset %d size %d: %w", offset, localChunk, err)
 		}
@@ -1218,7 +1220,7 @@ func (o *Object) uploadSession(ctx context.Context, parentID, name string) (uplo
 	return o.fs.uploadLink(ctx, parentID, encName)
 }
 
-func (o *Object) upload(ctx context.Context, uploadKey string, chunk io.Reader, fullSize int64, offset int64, chunkSize int64, options ...fs.OpenOption) (err error) {
+func (o *Object) upload(ctx context.Context, uploadKey string, chunk io.ReadSeeker, fullSize int64, offset int64, chunkSize int64, options ...fs.OpenOption) (err error) {
 	opts := rest.Opts{
 		Method:        "POST",
 		RootURL:       fmt.Sprintf(uploadURL, o.fs.opt.Host) + uploadKey,
@@ -1231,6 +1233,11 @@ func (o *Object) upload(ctx context.Context, uploadKey string, chunk io.Reader,
 	var fileID string
 
 	err = o.fs.pacer.Call(func() (bool, error) {
+		// Rewind the chunk so a retry re-sends it in full
+		_, err := chunk.Seek(0, io.SeekStart)
+		if err != nil {
+			return false, err
+		}
 		resp, err := o.fs.srv.CallJSON(ctx, &opts, nil, &fileID)
 		return shouldRetry(ctx, resp, err)
 	})