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.