I mistakenly removed the call to eventfd->poll() while I was actually
intending to remove the return value...

Calling evenfd->poll() will hook cgroup_event_wake() to the poll
waitqueue, which will be called to unregister eventfd when rmdir a
cgroup or close eventfd.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>