benchmark : changed `fstat` to `fstatSync`

PR-URL: https://github.com/nodejs/node/pull/36206
Fixes: https://github.com/nodejs/node/issues/36199


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>