netfilter: nf_log_syslog: Unset bridge logger in pernet exit

Without this, a stale pointer remains in pernet loggers after module
unload causing a kernel oops during dereference. Easily reproduced by:

| # modprobe nf_log_syslog
| # rmmod nf_log_syslog
| # cat /proc/net/netfilter/nf_log

Fixes: 77ccee96 ("netfilter: nf_log_bridge: merge with nf_log_syslog")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_log_syslog.c

@@ -1011,6 +1011,7 @@ static void __net_exit nf_log_syslog_net_exit(struct net *net)
 	nf_log_unset(net, &nf_arp_logger);
 	nf_log_unset(net, &nf_ip6_logger);
 	nf_log_unset(net, &nf_netdev_logger);
+	nf_log_unset(net, &nf_bridge_logger);
 }
 
 static struct pernet_operations nf_log_syslog_net_ops = {