bcache: fix error return value in memory shrink

In bch_mca_scan(), the return value should not be the number of freed btree
nodes, but the number of pages of freed btree nodes.
Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/btree.c

@@ -719,7 +719,7 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
 	}
 out:
 	mutex_unlock(&c->bucket_lock);
-	return freed;
+	return freed * c->btree_pages;
 }
 
 static unsigned long bch_mca_count(struct shrinker *shrink,