Kernel: VERIFY that nobody is holding lock in ~BlockerSet()

By the time we end up destroying a BlockerSet, we don't need to take
the internal spinlock. And nobody else should be holding it either.
So replace the SpinlockLocker with a VERIFY().

Kernel/Thread.h

@@ -398,7 +398,7 @@ public:
 
         virtual ~BlockerSet()
         {
-            SpinlockLocker lock(m_lock);
+            VERIFY(!m_lock.is_locked());
             VERIFY(m_blockers.is_empty());
         }