bcache: add a comment in super.c

This patch adds a line of code comment in super.c:register_bdev(), to
make code to be more comprehensible.
Signed-off-by: NColy Li <colyli@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -1291,6 +1291,7 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
 	pr_info("registered backing device %s", dc->backing_dev_name);
 
 	list_add(&dc->list, &uncached_devices);
+	/* attach to a matched cache set if it exists */
 	list_for_each_entry(c, &bch_cache_sets, list)
 		bch_cached_dev_attach(dc, c, NULL);