nvme: Eliminate RECOVERY_FAILED state

While it seemed like a good idea to have this state, we can do
everything we wanted with the state by checking ctrlr->is_failed since
that's set before we start failing the qpairs. Add some comments about
racing when we're failing the controller, though in practice I'm not
sure that kind of race could even be lost.

Sponsored by: Netflix
Reviewed by: chuck, gallatin, jhb
Differential Revision: https://reviews.freebsd.org/D42051