[kernel] Remove thread_data pointer from TCB

The TCB is always stored at a constant offset within the thread object.
So instead of carrying an extra pointer, just implement thread::from_tcb
to get the thread.

src/kernel/objects/thread.cpp

@@ -16,10 +16,17 @@ thread::thread(process &parent, uint8_t pri) :
 	TCB *tcbp = tcb();
 	tcbp->pml4 = parent.pml4();
 	tcbp->priority = pri;
-	tcbp->thread_data = this;
 	set_state(state::ready);
 }
 
+thread *
+thread::from_tcb(TCB *tcb)
+{
+	static ptrdiff_t offset =
+		-1 * static_cast<ptrdiff_t>(offsetof(thread, m_tcb));
+	return reinterpret_cast<thread*>(kutil::offset_pointer(tcb, offset));
+}
+
 void
 thread::wait_on_signals(kobject *obj, j6_signal_t signals)
 {