AK+Kernel: Make IntrusiveList capable of holding non-raw pointers

This should allow creating intrusive lists that have smart pointers,
while remaining free (compared to the impl before this commit) when
holding raw pointers :^)
As a sidenote, this also adds a `RawPtr<T>` type, which is just
equivalent to `T*`.
Note that this does not actually use such functionality, but is only
expected to pave the way for #6369, to replace NonnullRefPtrVector<T>
with intrusive lists.

As it is with zero-cost things, this makes the interface a bit less nice
by requiring the type name of what an `IntrusiveListNode` holds (and
optionally its container, if not RawPtr), and also requiring the type of
the container (normally `RawPtr`) on the `IntrusiveList` instance.

Kernel/FileSystem/BlockBasedFileSystem.cpp

@@ -32,7 +32,7 @@
 namespace Kernel {
 
 struct CacheEntry {
-    IntrusiveListNode list_node;
+    IntrusiveListNode<CacheEntry> list_node;
     BlockBasedFS::BlockIndex block_index { 0 };
     u8* data { nullptr };
     bool has_data { false };
@@ -117,8 +117,8 @@ private:
     BlockBasedFS& m_fs;
     size_t m_entry_count { 10000 };
     mutable HashMap<BlockBasedFS::BlockIndex, CacheEntry*> m_hash;
-    mutable IntrusiveList<CacheEntry, &CacheEntry::list_node> m_clean_list;
-    mutable IntrusiveList<CacheEntry, &CacheEntry::list_node> m_dirty_list;
+    mutable IntrusiveList<CacheEntry, RawPtr<CacheEntry>, &CacheEntry::list_node> m_clean_list;
+    mutable IntrusiveList<CacheEntry, RawPtr<CacheEntry>, &CacheEntry::list_node> m_dirty_list;
     KBuffer m_cached_block_data;
     KBuffer m_entries;
     bool m_dirty { false };

Kernel/Process.h

@@ -120,7 +120,7 @@ protected:
     mode_t m_umask { 022 };
     VirtualAddress m_signal_trampoline;
     Atomic<u32> m_thread_count { 0 };
-    IntrusiveList<Thread, &Thread::m_process_thread_list_node> m_thread_list;
+    IntrusiveList<Thread, RawPtr<Thread>, &Thread::m_process_thread_list_node> m_thread_list;
     u8 m_termination_status { 0 };
     u8 m_termination_signal { 0 };
 };

Kernel/Scheduler.cpp

@@ -73,7 +73,7 @@ Atomic<bool> g_finalizer_has_work { false };
 READONLY_AFTER_INIT static Process* s_colonel_process;
 
 struct ThreadReadyQueue {
-    IntrusiveList<Thread, &Thread::m_ready_queue_node> thread_list;
+    IntrusiveList<Thread, RawPtr<Thread>, &Thread::m_ready_queue_node> thread_list;
 };
 static SpinLock<u8> g_ready_queues_lock;
 static u32 g_ready_queues_mask;

Kernel/Thread.h

@@ -88,7 +88,7 @@ class Thread
     friend class Process;
     friend class ProtectedProcessBase;
     friend class Scheduler;
-    friend class ThreadReadyQueue;
+    friend struct ThreadReadyQueue;
 
     static SpinLock<u8> g_tid_map_lock;
     static HashMap<ThreadID, Thread*>* g_tid_map;
@@ -1129,7 +1129,7 @@ public:
 private:
     Thread(NonnullRefPtr<Process>, NonnullOwnPtr<Region> kernel_stack_region);
 
-    IntrusiveListNode m_process_thread_list_node;
+    IntrusiveListNode<Thread> m_process_thread_list_node;
     int m_runnable_priority { -1 };
 
     friend class WaitQueue;
@@ -1202,7 +1202,7 @@ private:
     TSS m_tss {};
     TrapFrame* m_current_trap { nullptr };
     u32 m_saved_critical { 1 };
-    IntrusiveListNode m_ready_queue_node;
+    IntrusiveListNode<Thread> m_ready_queue_node;
     Atomic<u32> m_cpu { 0 };
     u32 m_cpu_affinity { THREAD_AFFINITY_DEFAULT };
     u32 m_ticks_left { 0 };

Kernel/WorkQueue.h

@@ -75,7 +75,7 @@ public:
 
 private:
     struct WorkItem {
-        IntrusiveListNode m_node;
+        IntrusiveListNode<WorkItem> m_node;
         void (*function)(void*);
         void* data;
         void (*free_data)(void*);
@@ -86,7 +86,7 @@ private:
 
     RefPtr<Thread> m_thread;
     WaitQueue m_wait_queue;
-    IntrusiveList<WorkItem, &WorkItem::m_node> m_items;
+    IntrusiveList<WorkItem, RawPtr<WorkItem>, &WorkItem::m_node> m_items;
     SpinLock<u8> m_lock;
 };