cgroup: restore the call to eventfd->poll()

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>

kernel/cgroup.c

@@ -3882,6 +3882,8 @@ static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
 	if (ret)
 		goto fail;
 
+	efile->f_op->poll(efile, &event->pt);
+
 	/*
 	 * Events should be removed after rmdir of cgroup directory, but before
 	 * destroying subsystem state objects. Let's take reference to cgroup