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/Process.h

@@ -112,7 +112,7 @@ class Process
 public:
     inline static Process* current()
     {
-        auto current_thread = Processor::current().current_thread();
+        auto current_thread = Processor::current_thread();
         return current_thread ? &current_thread->process() : nullptr;
     }