[NETFILTER]: nf_conntrack: support for layer 3 protocol load on demand

x_tables matches and targets that require nf_conntrack_ipv[4|6] to work
don't have enough information to load on demand these modules. This
patch introduces the following changes to solve this issue:

o nf_ct_l3proto_try_module_get: try to load the layer 3 connection
tracker module and increases the refcount.
o nf_ct_l3proto_module put: drop the refcount of the module.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/xt_helper.c

diff --git a/net/netfilter/xt_helper.c b/net/netfilter/xt_helper.c
index 7d2d68b..101f000 100644
--- a/net/netfilter/xt_helper.c
+++ b/net/netfilter/xt_helper.c
@@ -144,15 +144,31 @@
 {
 	struct xt_helper_info *info = matchinfo;
 
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+	if (nf_ct_l3proto_try_module_get(match->family) < 0) {
+		printk(KERN_WARNING "can't load nf_conntrack support for "
+				    "proto=%d\n", match->family);
+		return 0;
+	}
+#endif
 	info->name[29] = '\0';
 	return 1;
 }
 
+static void
+destroy(const struct xt_match *match, void *matchinfo, unsigned int matchsize)
+{
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+	nf_ct_l3proto_module_put(match->family);
+#endif
+}
+
 static struct xt_match helper_match = {
 	.name		= "helper",
 	.match		= match,
 	.matchsize	= sizeof(struct xt_helper_info),
 	.checkentry	= check,
+	.destroy	= destroy,
 	.family		= AF_INET,
 	.me		= THIS_MODULE,
 };
@@ -161,6 +177,7 @@
 	.match		= match,
 	.matchsize	= sizeof(struct xt_helper_info),
 	.checkentry	= check,
+	.destroy	= destroy,
 	.family		= AF_INET6,
 	.me		= THIS_MODULE,
 };