cgroup: remove cgrp->kn check in css_populate_dir()

cgroup_create() creates cgrp and assigns the kernfs_node to cgrp->kn,
then cgroup_mkdir() populates base and csses cft file by calling
css_populate_dir() and cgroup_apply_control_enable() with a valid
cgrp->kn. Check for NULL cgrp->kn, will always be false, remove it.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>