fs: make writeFile consistent with readFile wrt fd

As it is, `readFile` always reads from the current position of the file,
if a file descriptor is used. But `writeFile` always writes from the
beginning of the file.

This patch fixes this inconsistency by making `writeFile` also to write
from the current position of the file when used with a file descriptor.

PR-URL: https://github.com/nodejs/node/pull/23709


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>