TUN_METADATA_OFFSET
((void *)((flow_key)->tun_opts + TUN_METADATA_OFFSET(opt_len)))
opt_key_offset = TUN_METADATA_OFFSET(nla_len(a));
opt_key_offset = TUN_METADATA_OFFSET(sizeof(opts));
opt_key_offset = TUN_METADATA_OFFSET(nla_len(a));