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/xfrm4_mode_tunnel.c b/net/ipv4/xfrm4_mode_tunnel.c
index 7135279..3444f3b 100644
--- a/net/ipv4/xfrm4_mode_tunnel.c
+++ b/net/ipv4/xfrm4_mode_tunnel.c
@@ -28,7 +28,7 @@
  */
 static int xfrm4_mode_tunnel_output(struct xfrm_state *x, struct sk_buff *skb)
 {
-	struct dst_entry *dst = skb->dst;
+	struct dst_entry *dst = skb_dst(skb);
 	struct iphdr *top_iph;
 	int flags;
 
@@ -41,7 +41,7 @@
 	top_iph->ihl = 5;
 	top_iph->version = 4;
 
-	top_iph->protocol = xfrm_af2proto(skb->dst->ops->family);
+	top_iph->protocol = xfrm_af2proto(skb_dst(skb)->ops->family);
 
 	/* DS disclosed */
 	top_iph->tos = INET_ECN_encapsulate(XFRM_MODE_SKB_CB(skb)->tos,