As struct btrfs_disk_super is being passed, so it can get devid
the same way its parent does.
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>