tools: apply custom buffer lint rule to /lib only

The lint rule is there to avoid a circular-dependency issue that only
applies to `/lib`. In preparation for linting `/benchmark`, apply that
rule to `/lib` only to avoid churn in `/benchmark`.

Refs: https://github.com/nodejs/node/issues/3983#issuecomment-158956113
PR-URL: https://github.com/nodejs/node/pull/5371


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>