netfilter: cttimeout: fix dependency with l4protocol conntrack module

This patch introduces nf_conntrack_l4proto_find_get() and
nf_conntrack_l4proto_put() to fix module dependencies between
timeout objects and l4-protocol conntrack modules.

Thus, we make sure that the module cannot be removed if it is
used by any of the cttimeout objects.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/include/net/netfilter/nf_conntrack_timeout.h b/include/net/netfilter/nf_conntrack_timeout.h
index 0e04db4..34ec89f 100644
--- a/include/net/netfilter/nf_conntrack_timeout.h
+++ b/include/net/netfilter/nf_conntrack_timeout.h
@@ -15,7 +15,7 @@
 	atomic_t		refcnt;
 	char			name[CTNL_TIMEOUT_NAME_MAX];
 	__u16			l3num;
-	__u8			l4num;
+	struct nf_conntrack_l4proto *l4proto;
 	char			data[0];
 };