Kernel: Fix a few Thread::block related races

We need to have a Thread lock to protect threading related
operations, such as Thread::m_blocker which is used in
Thread::block.

Also, if a Thread::Blocker indicates that it should be
unblocking immediately, don't actually block the Thread
and instead return immediately in Thread::block.

Kernel/FileSystem/Plan9FileSystem.cpp

@@ -498,7 +498,7 @@ KResult Plan9FS::read_and_dispatch_one_message()
     return KSuccess;
 }
 
-bool Plan9FS::Blocker::should_unblock(Thread&, time_t, long)
+bool Plan9FS::Blocker::should_unblock(Thread&)
 {
     if (m_completion.completed)
         return true;