netfilter: ipset: use nfnl_mutex_is_locked

Check that we really hold nfnl mutex here instead of relying on correct
usage alone.

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>