Lock the new task's scheduler when creating a task

Previously we were locking the spawning task's scheduler. I couldn't
see that that was protecting anything. The newborn_task list in the new task's
scheduler though was unprotected from concurrent access. So now we're locking
the new task's scheduler.

src/rt/rust_kernel.cpp

@@ -135,7 +135,9 @@ int rust_kernel::start_task_threads()
 
 rust_task *
 rust_kernel::create_task(rust_task *spawner, const char *name) {
-    return threads[rand(&rctx) % num_threads]->create_task(spawner, name);
+    rust_scheduler *thread = threads[rand(&rctx) % num_threads];
+    scoped_lock with(thread->lock);
+    return thread->create_task(spawner, name);
 }
 
 void rust_kernel::wakeup_schedulers() {