netfilter: ipset: Fix calling ip_set() macro at dumping

The ip_set() macro is called when either ip_set_ref_lock held only
or no lock/nfnl mutex is held at dumping. Take this into account
properly. Also, use Pablo's suggestion to use rcu_dereference_raw(),
the ref_netlink protects the set.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>