netfilter: use a linked list of loggers

This patch modifies nf_log to use a linked list of loggers for each
protocol. This list of loggers is read and write protected with a
mutex.

This patch separates registration and binding. To be used as
logging module, a module has to register calling nf_log_register()
and to bind to a protocol it has to call nf_log_bind_pf().
This patch also converts the logging modules to the new API. For nfnetlink_log,
it simply switchs call to register functions to call to bind function and
adds a call to nf_log_register() during init. For other modules, it just
remove a const flag from the logger structure and replace it with a
__read_mostly.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/ipv6/netfilter/ip6t_LOG.c

@@ -477,7 +477,7 @@ static struct xt_target log_tg6_reg __read_mostly = {
 	.me 		= THIS_MODULE,
 };
 
-static const struct nf_logger ip6t_logger = {
+static struct nf_logger ip6t_logger __read_mostly = {
 	.name		= "ip6t_LOG",
 	.logfn		= &ip6t_log_packet,
 	.me		= THIS_MODULE,