block: really clone the block cgroup in bio_clone_blkg_association

bio_clone_blkg_association is supposed to clone the associatation, but
actually ends up doing a search with a tryget.  As we know we have a
reference on the source cgroup just get an unconditional additional
reference to it and call it a day.  That also removes the need for
a RCU critical section.
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -1737,12 +1737,12 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
  */
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
-	rcu_read_lock();
-
-	if (src->bi_blkg)
-		__bio_associate_blkg(dst, src->bi_blkg);
-
-	rcu_read_unlock();
+	if (src->bi_blkg) {
+		if (dst->bi_blkg)
+			blkg_put(dst->bi_blkg);
+		blkg_get(src->bi_blkg);
+		dst->bi_blkg = src->bi_blkg;
+	}
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
 #endif /* CONFIG_BLK_CGROUP */