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

net/netfilter/ipset/ip_set_core.c

@@ -57,7 +57,7 @@ MODULE_ALIAS_NFNL_SUBSYS(NFNL_SUBSYS_IPSET);
 
 /* When the nfnl mutex is held: */
 #define ip_set_dereference(p)		\
-	rcu_dereference_protected(p, 1)
+	rcu_dereference_protected(p, lockdep_nfnl_is_held(NFNL_SUBSYS_IPSET))
 #define ip_set(inst, id)		\
 	ip_set_dereference((inst)->ip_set_list)[id]