Kernel: Scheduler donations need to verify that the beneficiary is valid.

Add a Thread::is_thread(void*) helper that we can use to check that the
incoming donation beneficiary is a valid thread. The O(n) here is a bit sad
and we should eventually rethink the process/thread table data structures.

Kernel/Thread.h

@@ -34,6 +34,7 @@ public:
     static void finalize_dying_threads();
 
     static Vector<Thread*> all_threads();
+    static bool is_thread(void*);
 
     int tid() const { return m_tid; }
     int pid() const;