filer: consistent timestamp handling in sequential read path

Use max(ts, task.chunk.ModifiedTsNs) in sequential path to match
parallel path behavior. Also update ts before error check so that
on failure, the returned timestamp reflects the max of all chunks
processed so far.

weed/filer/reader_at.go

@@ -244,12 +244,12 @@ func (c *ChunkReadAt) doReadAt(ctx context.Context, p []byte, offset int64) (n i
 	if len(tasks) <= 1 || c.readerPattern.IsRandomMode() {
 		for _, task := range tasks {
 			copied, readErr := c.readChunkSliceAt(ctx, p[task.bufferStart:task.bufferEnd], task.chunk, nil, task.chunkOffset)
+			ts = max(ts, task.chunk.ModifiedTsNs)
 			if readErr != nil {
 				glog.Errorf("fetching chunk %+v: %v\n", task.chunk, readErr)
-				return n + copied, task.chunk.ModifiedTsNs, readErr
+				return n + copied, ts, readErr
 			}
 			n += copied
-			ts = task.chunk.ModifiedTsNs
 		}
 	} else {
 		// Parallel chunk fetching for multiple chunks