net: skb->dst accessors

Define three accessors to get/set dst attached to a skb

struct dst_entry *skb_dst(const struct sk_buff *skb)

void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)

void skb_dst_drop(struct sk_buff *skb)
This one should replace occurrences of :
dst_release(skb->dst)
skb->dst = NULL;

Delete skb->dst field

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 79c39dc..416fc4c 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2202,7 +2202,7 @@
 	/* Reserve space for headers. */
 	skb_reserve(skb, MAX_TCP_HEADER);
 
-	skb->dst = dst_clone(dst);
+	skb_dst_set(skb, dst_clone(dst));
 
 	mss = dst_metric(dst, RTAX_ADVMSS);
 	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)