[NETFILTER]: nf_conntrack: add tuplehash l3num/protonum accessors

Add accessors for l3num and protonum and get rid of some overly long
expressions.

Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/netfilter/xt_connlimit.c b/net/netfilter/xt_connlimit.c
index 0ca9fe9..2e89a00 100644
--- a/net/netfilter/xt_connlimit.c
+++ b/net/netfilter/xt_connlimit.c
@@ -72,9 +72,7 @@
 
 static inline bool already_closed(const struct nf_conn *conn)
 {
-	u_int16_t proto = conn->tuplehash[0].tuple.dst.protonum;
-
-	if (proto == IPPROTO_TCP)
+	if (nf_ct_protonum(conn) == IPPROTO_TCP)
 		return conn->proto.tcp.state == TCP_CONNTRACK_TIME_WAIT;
 	else
 		return 0;