fs: fix writeFile[Sync] for non-seekable files

Completely disables the use of positioned writes at
writeFile and writeFileSync, which allows it to work
with non-seekable files.

Fixes: https://github.com/nodejs/node/issues/31926

PR-URL: https://github.com/nodejs/node/pull/32006


Reviewed-By: Anna Henningsen <anna@addaleax.net>