Kernel: Use kfree_sized() in KString

Kernel/KString.cpp

@@ -59,7 +59,8 @@ OwnPtr<KString> KString::try_clone() const
 
 void KString::operator delete(void* string)
 {
-    kfree(string);
+    size_t allocation_size = sizeof(KString) + (sizeof(char) * static_cast<KString*>(string)->m_length) + sizeof(char);
+    kfree_sized(string, allocation_size);
 }
 
 }