[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)
 {

src/kernel/objects/thread.h

@@ -27,8 +27,6 @@ struct TCB
 
 	uint32_t time_left;
 	uint64_t last_ran;
-
-	thread *thread_data;
 };
 
 using tcb_list = kutil::linked_list<TCB>;
@@ -47,6 +45,9 @@ public:
 		none     = 0x00
 	};
 
+	/// Get the pointer to the thread object containing this TCB
+	static thread * from_tcb(TCB *tcb);
+
 	/// Get the `ready` state of the thread.
 	/// \returns True if the thread is ready to execute.
 	inline bool ready() const { return has_state(state::ready); }