net: fix bufferSize

bufferSize should only look at writableLength otherwise it will
always show more than what is actually pending.

PR-URL: https://github.com/nodejs/node/pull/34088
Refs: https://github.com/nodejs/node/issues/34078


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>