lib: change == to === in linkedlist

Also removed a TODO comment that is no longer viable and left a note
about the potentially confusing property naming convention for future
readers.

PR-URL: https://github.com/nodejs/node/pull/9362


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>