Minimize scheduler locking on task creation

This takes my CPU utilization on task-perf-spawnalot from 35% to 55%

src/rt/rust_scheduler.cpp

@@ -272,7 +272,9 @@ rust_scheduler::create_task(rust_task *spawner, const char *name) {
         task->pin(spawner->pinned_on);
         task->on_wakeup(spawner->_on_wakeup);
     }
+    lock.lock();
     newborn_tasks.append(task);
+    lock.unlock();
 
     sync::increment(kernel->live_tasks);