fs: fix length option being ignored during read()

Currently, `length` in an options object is ignored.

lib/internal/fs/promises.js

@@ -520,7 +520,7 @@ async function read(handle, bufferOrOptions, offset, length, position) {
       buffer = Buffer.alloc(16384);
     }
     offset = bufferOrOptions.offset || 0;
-    length = buffer.byteLength;
+    length = bufferOrOptions.length || buffer.byteLength;
     position = bufferOrOptions.position ?? null;
   }