[kernel] Fix scheduler deadlocks

The scheduler queue locks could deadlock if the timer fired before the
scoped lock destructor ran. Also, reduce lock contention by letting only
one CPU steal work at a time.

src/kernel/scheduler.h

@@ -97,7 +97,7 @@ private:
 	// TODO: lol a real clock
 	uint64_t m_clock = 0;
 
-	kutil::spinlock m_steal_lock;
+	unsigned m_steal_turn = 0;
 	static scheduler *s_instance;
 };