cgroup: fix coccinelle warnings

kernel/cgroup.c:2256:1-3: WARNING: PTR_RET can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: coccinelle/api/ptr_ret.cocci
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -2171,9 +2171,7 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
 	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
 				  cgroup_file_mode(cft), 0, cft->kf_ops, cft,
 				  NULL, false, key);
-	if (IS_ERR(kn))
-		return PTR_ERR(kn);
-	return 0;
+	return PTR_ERR_OR_ZERO(kn);
 }
 
 /**