src: clean up `req.bytes` tracking

Simply always tell the caller how many bytes were written, rather
than letting them track it.

In the case of writing a string, also keep track of the bytes
written by the earlier `DoTryWrite()`.

Refs: https://github.com/nodejs/node/issues/19562

PR-URL: https://github.com/nodejs/node/pull/19551


Reviewed-By: James M Snell <jasnell@gmail.com>