This patches fixes various bugs I hit in the recently posted container
patches.
1. If a subsystem registers with fork/exit hook during bootup (much
before rcu is initialized), then the resulting synchronize_rcu() in
container_register_subsys() hangs. Avoid this by not calling
synchronize_rcu() if we arent fully booted yet.
2. If cpuset_create fails() for some reason, then the resulting
call to cpuset_destroy can trip. Avoid this by initializing
container->...->cpuset pointer to NULL in cpuset_create().
3. container_rmdir->cpuset_destroy->update_flag can deadlock on
container_lock(). Avoid this by introducing __update_flag, which
doesnt take container_lock().