devcg: remove parent_cgroup.

In devcgroup_css_alloc(), there is no longer need for parent_cgroup.
bd2953eb("devcg: propagate local changes down the hierarchy") made
the variable parent_cgroup redundant. This patch removes parent_cgroup
from devcgroup_css_alloc().

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>