[#970] fstree: Move temporary path handling in a separate function

Allow to easier test different implementations.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -357,9 +357,12 @@ func (t *FSTree) Put(ctx context.Context, prm common.PutPrm) (common.PutRes, err
 	}
 
 	size = len(prm.RawData)
+	return common.PutRes{StorageID: []byte{}}, t.writeData(p, prm.RawData)
+}
+
+func (t *FSTree) writeData(p string, data []byte) error {
 	tmpPath := p + "#" + strconv.FormatUint(t.suffix.Add(1), 10)
-	err = t.writeAndRename(tmpPath, p, prm.RawData)
-	return common.PutRes{StorageID: []byte{}}, err
+	return t.writeAndRename(tmpPath, p, data)
 }
 
 // writeAndRename opens tmpPath exclusively, writes data to it and renames it to p.