lib: Change ot_util_variant_map helpers to consistently sink refs

lib: Change ot_util_variant_map helpers to consistently sink refs

This is similar to changes Krzesimir has been doing recently - we
really don't need the ergonomics of floating refs since we have
autocleanups.

We should continue to change most of our code to sink refs.

Specifically here it was pretty broken that the `_map()` API was
sinking but the other two weren't, and this broke some refactoring I
was trying to do later.

Closes: #317
Approved by: jlebon