+/*
+ * When operations want to vput() a union node yet retain a lock on
+ * the upper vnode (say, to do some further operations like link(),
+ * mkdir(), ...), they set UN_KLOCK on the union node, then call
+ * vput() which calls VOP_UNLOCK() and comes here. union_unlock()
+ * unlocks the union node (leaving the upper vnode alone), clears the
+ * KLOCK flag, and then returns to vput(). The caller then does whatever
+ * is left to do with the upper vnode, and ensures that it gets unlocked.
+ *
+ * If UN_KLOCK isn't set, then the upper vnode is unlocked here.
+ */