Revert "get rid of unneeded mut and refactor addref (solana-labs#19594)"

This reverts commit ca1b04e.

runtime/src/accounts_index.rs

@@ -190,7 +190,7 @@ impl<T: IsCached> WriteAccountMapEntry<T> {
         .build()
     }
 
-    pub fn slot_list(&self) -> &SlotList<T> {
+    pub fn slot_list(&mut self) -> &SlotList<T> {
         &*self.borrow_slot_list_guard()
     }
 
@@ -201,6 +201,10 @@ impl<T: IsCached> WriteAccountMapEntry<T> {
         self.with_slot_list_guard_mut(user)
     }
 
+    pub fn ref_count(&self) -> &AtomicU64 {
+        &self.borrow_owned_entry().ref_count
+    }
+
     // create an entry that is equivalent to this process:
     // 1. new empty (refcount=0, slot_list={})
     // 2. update(slot, account_info)
@@ -325,7 +329,7 @@ impl<T: IsCached> WriteAccountMapEntry<T> {
         });
         if addref {
             // If it's the first non-cache insert, also bump the stored ref count
-            self.borrow_owned_entry().add_un_ref(true);
+            self.ref_count().fetch_add(1, Ordering::Relaxed);
         }
     }
 }