fix lint warning

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

internal/vfs/vfs.go

@@ -1282,16 +1282,16 @@ func readFill(r io.Reader, buf []byte) (n int, err error) {
 	return n, err
 }
 
-func writeAtFull(w io.WriterAt, buf []byte, offset int64, count int) (int, error) {
+func writeAtFull(w io.WriterAt, buf []byte, offset int64, count int) error {
 	written := 0
 	for written < count {
 		n, err := w.WriteAt(buf[written:count], offset+int64(written))
 		written += n
 		if err != nil {
-			return written, err
+			return err
 		}
 	}
-	return written, nil
+	return nil
 }
 
 type bytesReaderWrapper struct {