bcache: add comments for kobj release callback routine

Bcache has several routines to release resources in implicit way, they
are called when the associated kobj released. This patch adds code
comments to notice when and which release callback will be called,
- When dc->disk.kobj released:
  void bch_cached_dev_release(struct kobject *kobj)
- When d->kobj released:
  void bch_flash_dev_release(struct kobject *kobj)
- When c->kobj released:
  void bch_cache_set_release(struct kobject *kobj)
- When ca->kobj released
  void bch_cache_release(struct kobject *kobj)

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -1172,6 +1172,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c,
 	return 0;
 }
 
+/* when dc->disk.kobj released */
 void bch_cached_dev_release(struct kobject *kobj)
 {
 	struct cached_dev *dc = container_of(kobj, struct cached_dev,
@@ -1324,6 +1325,7 @@ err:
 
 /* Flash only volumes */
 
+/* When d->kobj released */
 void bch_flash_dev_release(struct kobject *kobj)
 {
 	struct bcache_device *d = container_of(kobj, struct bcache_device,
@@ -1494,6 +1496,7 @@ bool bch_cache_set_error(struct cache_set *c, const char *fmt, ...)
 	return true;
 }
 
+/* When c->kobj released */
 void bch_cache_set_release(struct kobject *kobj)
 {
 	struct cache_set *c = container_of(kobj, struct cache_set, kobj);
@@ -2021,6 +2024,7 @@ err:
 
 /* Cache device */
 
+/* When ca->kobj released */
 void bch_cache_release(struct kobject *kobj)
 {
 	struct cache *ca = container_of(kobj, struct cache, kobj);