Kernel: Add a SpinLock to the WaitQueue

We need to be able to prevent a WaitQueue from being
modified by another CPU. So, add a SpinLock to it.

Because this pushes some other class over the 64 byte
limit, we also need to add another 128-byte bucket to
the slab allocator.

Kernel/WaitQueue.h

@@ -28,6 +28,7 @@
 
 #include <AK/Atomic.h>
 #include <AK/SinglyLinkedList.h>
+#include <Kernel/SpinLock.h>
 #include <Kernel/Thread.h>
 
 namespace Kernel {
@@ -46,6 +47,7 @@ public:
 private:
     typedef IntrusiveList<Thread, &Thread::m_wait_queue_node> ThreadList;
     ThreadList m_threads;
+    SpinLock<u32> m_lock;
 };
 
 }