Revert "buffer: convert offset & length to int properly"

This reverts commit ca37fa52.

A test provided by the commit fails on most (but not all) platforms on
CI.

PR-URL: https://github.com/nodejs/node/pull/9814
Ref: https://github.com/nodejs/node/pull/9492


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>