buffer: fix custom inspection with extra properties

This broke due to a recent change that prevents exposing inspect
internals. It now relies on the public API instead and should be a
bit more robust due to that.

PR-URL: https://github.com/nodejs/node/pull/27074


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>