Inactive incorrectly incremented

Inactive incorrectly incremented

It is possible to have two threads destroying an rbtdb at the same
time when detachnode() executes and removes the last reference to
a node between exiting being set to true for the node and testing
if the references are zero in maybe_free_rbtdb().  Move NODE_UNLOCK()
to after checking if references is zero to prevent detachnode()
changing the reference count too early.

(cherry picked from commit 859d2fdad6d1c6ff20083a4c463a929cbeb26438)