md-bitmap: create and destroy wb_info_pool with the change of bitmap

The write-behind attribute is part of bitmap, since bitmap
can be added/removed dynamically with the following.

1. mdadm --grow /dev/md0 --bitmap=none
2. mdadm --grow /dev/md0 --bitmap=internal --write-behind

So we need to destroy wb_info_pool in md_bitmap_destroy,
and create the pool before load bitmap.
Reviewed-by: NNeilBrown <neilb@suse.com>
Signed-off-by: NGuoqing Jiang <gqjiang@suse.com>
Signed-off-by: NSong Liu <songliubraving@fb.com>

drivers/md/md-bitmap.c

@@ -1790,6 +1790,8 @@ void md_bitmap_destroy(struct mddev *mddev)
 		return;
 
 	md_bitmap_wait_behind_writes(mddev);
+	mempool_destroy(mddev->wb_info_pool);
+	mddev->wb_info_pool = NULL;
 
 	mutex_lock(&mddev->bitmap_info.mutex);
 	spin_lock(&mddev->lock);
@@ -1900,10 +1902,14 @@ int md_bitmap_load(struct mddev *mddev)
 	sector_t start = 0;
 	sector_t sector = 0;
 	struct bitmap *bitmap = mddev->bitmap;
+	struct md_rdev *rdev;
 
 	if (!bitmap)
 		goto out;
 
+	rdev_for_each(rdev, mddev)
+		mddev_create_wb_pool(mddev, rdev, true);
+
 	if (mddev_is_clustered(mddev))
 		md_cluster_ops->load_bitmaps(mddev, mddev->bitmap_info.nodes);