cgroup: remove unnecessary empty check when enabling threaded mode

cgroup_enable_threaded() checks that the cgroup doesn't have any tasks
or children and fails the operation if so.  This test is unnecessary
because the first part is already checked by
cgroup_can_be_thread_root() and the latter is unnecessary.  The latter
actually cause a behavioral oddity.  Please consider the following
hierarchy.  All cgroups are domains.

    A
   / \
  B   C
       \
        D

If B is made threaded, C and D becomes invalid domains.  Due to the no
children restriction, threaded mode can't be enabled on C.  For C and
D, the only thing the user can do is removal.

There is no reason for this restriction.  Remove it.

Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/cgroup-v2.txt

@@ -274,8 +274,9 @@ thread mode, the following conditions must be met.
 - As the cgroup will join the parent's resource domain.  The parent
   must either be a valid (threaded) domain or a threaded cgroup.
 
-- The cgroup must be empty.  No enabled controllers, child cgroups or
+- When the parent is an unthreaded domain, it must not have any domain
-  processes.
+  controllers enabled or populated domain children.  The root is
+  exempt from this requirement.
 
 Topology-wise, a cgroup can be in an invalid state.  Please consider
 the following toplogy::

kernel/cgroup/cgroup.c

@@ -3146,13 +3146,6 @@ static int cgroup_enable_threaded(struct cgroup *cgrp)
 	    !cgroup_can_be_thread_root(dom_cgrp))
 		return -EOPNOTSUPP;
 
-	/*
-	 * Allow enabling thread mode only on empty cgroups to avoid
-	 * implicit migrations and recursive operations.
-	 */
-	if (cgroup_has_tasks(cgrp) || css_has_online_children(&cgrp->self))
-		return -EBUSY;
-
 	/*
 	 * The following shouldn't cause actual migrations and should
 	 * always succeed.