device_cgroup: remove direct access to cgroup->children
device_cgroup: remove direct access to cgroup->children

Currently, devcg::has_children() directly tests cgroup->children for
list emptiness.  The field is not a published field and scheduled to
go away.  In addition, the test isn't strictly correct as devcg should
only care about children which are visible to userland.

This patch converts has_children() to use css_next_child() instead.
The subtle incorrectness is noted and will be dealt with later.

Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>
Acked-by: Aristeu Rozanski <78e305742ac743ecf22d8139502576ac8709e0ad@redhat.com>
Acked-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>
Acked-by: Li Zefan <a8167b9d27aca9c6624754deb52c4c6420eded04@huawei.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs