Lockdep gets angry when we try to destroy an accepted conn workqueue
from within work in a listening conn's workqueue. It doesn't recognize
that they have a hierarchical relationship that maintains a consistent
order and we can't get at the workqueue lockdep_map to set subclasses.
We add a destroy workqueue which will have its own class.
Signed-off-by: Zach Brown <zab@versity.com>