net: handle a case when --empty net is set only for criu dump

The origin idea was to set --empty net for criu dump and criu restore,
but before cde33dcb ("empty-ns: Don't C/R iptables too (v2)"),
criu restore worked without --empty net and we didn't notice that
docker doesn't set this option on restore.

After a small brainstorm, we decided that it is better to remove
this requirement. Docker has to set this option, but with this changes,
the docker issue will be less urgent.

https://github.com/checkpoint-restore/criu/issues/393