buffer: simplify code

This refactors some code for simplicity. It also removes a call
indirection used in the buffers custom inspect function.

PR-URL: https://github.com/nodejs/node/pull/25151


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>