bcache: add the missing comments for smp_mb()/smp_wmb()

Checkpatch.pl warns there are 2 locations of smp_mb() and smp_wmb()
without code comment. This patch adds the missing code comments for
these memory barrier calls.
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/closure.h

@@ -289,10 +289,12 @@ static inline void closure_init_stack(struct closure *cl)
 }
 
 /**
- * closure_wake_up - wake up all closures on a wait list.
+ * closure_wake_up - wake up all closures on a wait list,
+ *		     with memory barrier
  */
 static inline void closure_wake_up(struct closure_waitlist *list)
 {
+	/* Memory barrier for the wait list */
 	smp_mb();
 	__closure_wake_up(list);
 }

drivers/md/bcache/super.c

@@ -1136,11 +1136,11 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c,
 	list_move(&dc->list, &c->cached_devs);
 	calc_cached_dev_sectors(c);
 
-	smp_wmb();
 	/*
 	 * dc->c must be set before dc->count != 0 - paired with the mb in
 	 * cached_dev_get()
 	 */
+	smp_wmb();
 	refcount_set(&dc->count, 1);
 
 	/* Block writeback thread, but spawn it */