cgroup: add dummy css_put() for !CONFIG_CGROUPS

This will later be depended upon by the scheduled cgroup writeback
support.
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -943,6 +943,8 @@ struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
 
 #else /* !CONFIG_CGROUPS */
 
+struct cgroup_subsys_state;
+
 static inline int cgroup_init_early(void) { return 0; }
 static inline int cgroup_init(void) { return 0; }
 static inline void cgroup_fork(struct task_struct *p) {}
@@ -955,6 +957,8 @@ static inline int cgroupstats_build(struct cgroupstats *stats,
 	return -EINVAL;
 }
 
+static inline void css_put(struct cgroup_subsys_state *css) {}
+
 /* No cgroups - nothing to do */
 static inline int cgroup_attach_task_all(struct task_struct *from,
 					 struct task_struct *t)