Kernel: Make Thread::current smp-safe

Change Thread::current to be a static function and read using the fs
register, which eliminates a window between Processor::current()
returning and calling a function on it, which can trigger preemption
and a move to a different processor, which then causes operating
on the wrong object.

Kernel/Thread.h

@@ -87,7 +87,7 @@ class Thread
 public:
     inline static Thread* current()
     {
-        return Processor::current().current_thread();
+        return Processor::current_thread();
     }
 
     explicit Thread(NonnullRefPtr<Process>);