btrfs: undo writable superblocke when sprouting fails

When new device is being added to seed FS, seed FS is marked writable,
but when we fail to bring in the new device, we missed to undo the
writable part. This patch fixes it.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2496,6 +2496,8 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 	return ret;
 
 error_trans:
+	if (seeding_dev)
+		sb->s_flags |= MS_RDONLY;
 	btrfs_end_transaction(trans);
 	rcu_string_free(device->name);
 	btrfs_sysfs_rm_device_link(fs_info->fs_devices, device);