Check if the subvolume is the current default.

def _is_default(path, dest, name):
    '''
    Check if the subvolume is the current default.
    '''
    subvol_id = __salt__['btrfs.subvolume_show'](path)[name]['subvolume id']
    def_id = __salt__['btrfs.subvolume_get_default'](dest)['id']
    return subvol_id == def_id