tcp: __tcp_hdrlen() helper

tcp_hdrlen is wasteful if you already have a pointer to struct tcphdr.
This splits the size calculation into a helper function that can be
used if a struct tcphdr is already available.
Signed-off-by: NCraig Gallek <kraig@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -29,9 +29,14 @@ static inline struct tcphdr *tcp_hdr(const struct sk_buff *skb)
 	return (struct tcphdr *)skb_transport_header(skb);
 }
 
+static inline unsigned int __tcp_hdrlen(const struct tcphdr *th)
+{
+	return th->doff * 4;
+}
+
 static inline unsigned int tcp_hdrlen(const struct sk_buff *skb)
 {
-	return tcp_hdr(skb)->doff * 4;
+	return __tcp_hdrlen(tcp_hdr(skb));
 }
 
 static inline struct tcphdr *inner_tcp_hdr(const struct sk_buff *skb)