tcp: unalias tcp_skb_cb flags and ip_dsfield

struct tcp_skb_cb contains a "flags" field containing either tcp flags
or IP dsfield depending on context (input or output path)

Introduce ip_dsfield to make the difference clear and ease maintenance.
If later we want to save space, we can union flags/ip_dsfield

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -642,7 +642,8 @@ struct tcp_skb_cb {
 #define TCPCB_SACKED_RETRANS	0x02	/* SKB retransmitted		*/
 #define TCPCB_LOST		0x04	/* SKB is lost			*/
 #define TCPCB_TAGBITS		0x07	/* All tag bits			*/
-
+	__u8		ip_dsfield;	/* IPv4 tos or IPv6 dsfield	*/
+	/* 1 byte hole */
 #define TCPCB_EVER_RETRANS	0x80	/* Ever retransmitted frame	*/
 #define TCPCB_RETRANS		(TCPCB_SACKED_RETRANS|TCPCB_EVER_RETRANS)
 

net/ipv4/tcp_input.c

@@ -222,7 +222,7 @@ static inline void TCP_ECN_check_ce(struct tcp_sock *tp, const struct sk_buff *s
 	if (!(tp->ecn_flags & TCP_ECN_OK))
 		return;
 
-	switch (TCP_SKB_CB(skb)->flags & INET_ECN_MASK) {
+	switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
 	case INET_ECN_NOT_ECT:
 		/* Funny extension: if ECT is not set on a segment,
 		 * and we already seen ECT on a previous segment,

net/ipv4/tcp_ipv4.c

@@ -1677,7 +1677,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 				    skb->len - th->doff * 4);
 	TCP_SKB_CB(skb)->ack_seq = ntohl(th->ack_seq);
 	TCP_SKB_CB(skb)->when	 = 0;
-	TCP_SKB_CB(skb)->flags	 = iph->tos;
+	TCP_SKB_CB(skb)->ip_dsfield = ipv4_get_dsfield(iph);
 	TCP_SKB_CB(skb)->sacked	 = 0;
 
 	sk = __inet_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest);

net/ipv6/tcp_ipv6.c

@@ -1717,7 +1717,7 @@ static int tcp_v6_rcv(struct sk_buff *skb)
 				    skb->len - th->doff*4);
 	TCP_SKB_CB(skb)->ack_seq = ntohl(th->ack_seq);
 	TCP_SKB_CB(skb)->when = 0;
-	TCP_SKB_CB(skb)->flags = ipv6_get_dsfield(hdr);
+	TCP_SKB_CB(skb)->ip_dsfield = ipv6_get_dsfield(hdr);
 	TCP_SKB_CB(skb)->sacked = 0;
 
 	sk = __inet6_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest);