src: read exactly two tokens from Linux THP sysfs config

There was an unexpected and hard-to-spot issue here:
the /sys/kernel/mm/transparent_hugepage/enabled file contains three
entries, and the std::ifstream reader was reading two values on each
loop iteration, resulting in incorrect behaviour.

Fixes: https://github.com/nodejs/node/issues/37064

PR-URL: https://github.com/nodejs/node/pull/37065


Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>