Fix *ring* benchmarks

[ranweiler]

The ring benchmarks were broken to recent updates to the PBKDF2 and AEAD interfaces. The relevant commits are:

• https://github.com/briansmith/ring/commit/8a655e45fc0e7926db3932677a82e875bc559ce6
• https://github.com/briansmith/ring/commit/86377e6a34916f87ca890a0561bc2bf3ce5efd1a
• https://github.com/briansmith/ring/commit/a9bbd302bba0b3012407d3a084c3f2116b624bd0

This PR just updates the impacted function calls for ring.

[briansmith]

Thank you!