cgroup: rename cgroup_subsys_state->dput_work and its callback function

css (cgroup_subsys_state) will become RCU protected and there will be
two stages which require punting to work item during release.  To
prepare for using the work item for multiple times, rename
css->dput_work to css->destroy_work and css_dput_fn() to
css_free_work_fn() and move work item initialization from css init to
right before the actual usage.

This reorganization doesn't introduce any behavior change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -80,7 +80,7 @@ struct cgroup_subsys_state {
 	struct css_id __rcu *id;
 
 	/* Used to put @cgroup->dentry on the last css_put() */
-	struct work_struct dput_work;
+	struct work_struct destroy_work;
 };
 
 /* bits in struct cgroup_subsys_state flags field */