fixup! benchmark: split Buffer.byteLength benchmark

Co-authored-by: Robert Nagy <ronagy@icloud.com>

benchmark/buffers/buffer-bytelength-buffer.js

@@ -7,7 +7,7 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ n, len }) {
-  let data = Buffer.alloc(len * 16, 'a');
+  const data = Buffer.alloc(len * 16, 'a');
   const expected = Buffer.byteLength(data, 'buffer');
   let changed = false;
   bench.start();