btrfs-progs: tune: implement resume support for empty csum tree

We have a very rare chance to hit a fs with empty csum tree but still
has CHANGING_DATA_CSUM flag.

The window is very small, but it's still possible, so handle it by
jumping directly to metadata csum change.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

tune/change-csum.c

@@ -827,6 +827,13 @@ static int resume_data_csum_change(struct btrfs_fs_info *fs_info, u16 new_csum_t
 	if (ret == 0)
 		new_csum_found = true;
 
+	/*
+	 * No csum item found at all, this fs has empty csum tree.
+	 * Just go metadata change.
+	 */
+	if (!old_csum_found && !new_csum_found)
+		goto new_meta_csum;
+
 	/*
 	 * Only old csums exists. This can be one of the two cases:
 	 * - Only the csum change item inserted, no new csum generated.