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>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5a4408c..7008fcc 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -222,7 +222,7 @@
 	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,