netfilter: ipset: fix netiface set name overflow

attribute is copied to IFNAMSIZ-size stack variable,
but IFNAMSIZ is smaller than IPSET_MAXNAMELEN.

Fortunately nfnetlink needs CAP_NET_ADMIN.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>