Summary:
[Folly] Style tweaks to `max_align_v` calculation.

* Change ns from `folly::detail` since that can be crowded and many other folly libraries may include this header.
* Extract a 2-param `max` alias.
* Fix an incorrectly hardcoded `size_t`.

Reviewed By: WillerZ

Differential Revision: D5468758

fbshipit-source-id: 14a3f67323020a3cfce2b3b46f5f64f3e6125027