Add destructor for the rib subscription system to simplify users code.

Subscriptions are planned to be used by modules such as route lookup engines.
In that case that's the module task to properly unsibscribe before detach.
However, the in-kernel customer - inet6 wants to track default route changes.
To avoid having inet6 store per-fib subscriptions, handle automatic
destruction internally.

Differential Revision: https://reviews.freebsd.org/D25614