netfilter: ipset: Missing nfnl_lock()/nfnl_unlock() is added to ip_set_net_exit()

Patch "netfilter: ipset: use nfnl_mutex_is_locked" is added the real
mutex locking check, which revealed the missing locking in ip_set_net_exit().
Signed-off-by: NJozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Reported-by: syzbot+36b06f219f2439fe62e1@syzkaller.appspotmail.com
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipset/ip_set_core.c

@@ -2078,6 +2078,7 @@ ip_set_net_exit(struct net *net)
 
 	inst->is_deleted = true; /* flag for ip_set_nfnl_put */
 
+	nfnl_lock(NFNL_SUBSYS_IPSET);
 	for (i = 0; i < inst->ip_set_max; i++) {
 		set = ip_set(inst, i);
 		if (set) {
@@ -2085,6 +2086,7 @@ ip_set_net_exit(struct net *net)
 			ip_set_destroy_set(set);
 		}
 	}
+	nfnl_unlock(NFNL_SUBSYS_IPSET);
 	kfree(rcu_dereference_protected(inst->ip_set_list, 1));
 }