fs: fix async iterator partial writes

fix an issue where chunks might be partially written when
using writeFile with an async iterator.

PR-URL: https://github.com/nodejs/node/pull/38615


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>