Revert "New invariant for dlist (#7294)" (#7301)

This reverts commit cf4d0e7.

src/util/dlist.h

@@ -155,23 +155,15 @@ class dll_base {
         elem->init(elem);
     }
 
-    bool invariant() const { 
-        auto* e = this; 
-        const T* slow = static_cast<const T*>(this); 
-        const T* fast = m_next; 
-        bool looped = false; 
-        // m_next of each node should point back to m_prev of the following node,
-        // and m_prev of each node should point forward to m_next of the preceding node.
-        while (slow != fast) { 
-            if (fast->m_prev->m_next != fast || fast->m_next->m_prev != fast) 
-                return false;             
-            fast = fast->m_next; 
-            looped = looped || (fast == static_cast<const T*>(this)); 
-            if (!looped && fast == m_next)  
-                // We should be able to traverse back to the starting node. 
-                return false;             
-        } 
-        return true; 
+    bool invariant() const {
+        auto* e = this;
+        do {
+            if (e->m_next->m_prev != e)
+                return false;
+            e = e->m_next;
+        }
+        while (e != this);
+        return true;
     }
 
     static bool contains(T const* list, T const* elem) {