Currently we save only attributes with non-zero values. For example,
a default value for IFLA_IPTUN_PROTO is IPPROTO_IPV6 (41), so we have to
save even attributes with zero values.

https://github.com/checkpoint-restore/criu/issues/445

Fixes: 4a044e6a ("net: Dump regular sit device")
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>