md: use new helper md_bitmap_get_stats() in update_array_info()

There are no functional changes, avoid dereferencing bitmap directly to
prepare inventing a new bitmap.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-4-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -7577,15 +7577,17 @@ static int update_array_info(struct mddev *mddev, mdu_array_info_t *info)
 			if (rv)
 				md_bitmap_destroy(mddev);
 		} else {
-			/* remove the bitmap */
-			if (!mddev->bitmap) {
-				rv = -ENOENT;
+			struct md_bitmap_stats stats;
+
+			rv = md_bitmap_get_stats(mddev->bitmap, &stats);
+			if (rv)
 				goto err;
-			}
-			if (mddev->bitmap->storage.file) {
+
+			if (stats.file) {
 				rv = -EINVAL;
 				goto err;
 			}
+
 			if (mddev->bitmap_info.nodes) {
 				/* hold PW on all the bitmap lock */
 				if (md_cluster_ops->lock_all_bitmaps(mddev) <= 0) {