Kernel: Stop using *LockRefPtr for Thread

These were stored in a bunch of places. The main one that's a bit iffy
is the Mutex::m_holder one, which I'm going to simplify in a subsequent
commit.

In Plan9FS and WorkQueue, we can't make the NNRPs const due to
initialization order problems. That's probably doable with further
cleanup, but left as an exercise for our future selves.

Before starting this, I expected the thread blockers to be a problem,
but as it turns out they were super straightforward (for once!) as they
don't mutate the thread after initiating a block, so they can just use
simple const-ified NNRPs.

Kernel/Locking/Mutex.h

@@ -101,7 +101,7 @@ private:
     // the lock is unlocked, it just means we don't know which threads hold it.
     // When locked exclusively, this is always the one thread that holds the
     // lock.
-    LockRefPtr<Thread> m_holder;
+    RefPtr<Thread> m_holder;
     size_t m_shared_holders { 0 };
 
     struct BlockedThreadLists {