refactor: CTxMemPool::UpdateChild() requires CTxMemPool::cs lock

No change in behavior, the lock is already held at call sites.

src/txmempool.cpp

@@ -968,6 +968,7 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, bool validFeeEstimat
 
 void CTxMemPool::UpdateChild(txiter entry, txiter child, bool add)
 {
+    AssertLockHeld(cs);
     setEntries s;
     if (add && mapLinks[entry].children.insert(child).second) {
         cachedInnerUsage += memusage::IncrementalDynamicUsage(s);

src/txmempool.h

@@ -569,7 +569,7 @@ private:
     txlinksMap mapLinks;
 
     void UpdateParent(txiter entry, txiter parent, bool add) EXCLUSIVE_LOCKS_REQUIRED(cs);
-    void UpdateChild(txiter entry, txiter child, bool add);
+    void UpdateChild(txiter entry, txiter child, bool add) EXCLUSIVE_LOCKS_REQUIRED(cs);
 
     std::vector<indexed_transaction_set::const_iterator> GetSortedDepthAndScore() const EXCLUSIVE_LOCKS_REQUIRED(cs);