[XFRM]: esp: fix skb_tail_pointer conversion bug

Fix incorrect switch of "trailer" skb by "skb" during skb_tail_pointer
conversion:

-       *(u8*)(trailer->tail - 1) = top_iph->protocol;
+       *(skb_tail_pointer(skb) - 1) = top_iph->protocol;

-       *(u8 *)(trailer->tail - 1) = *skb_network_header(skb);
+       *(skb_tail_pointer(skb) - 1) = *skb_network_header(skb);

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/esp4.c

@@ -64,7 +64,7 @@ static int esp_output(struct xfrm_state *x, struct sk_buff *skb)
 	esph = (struct ip_esp_hdr *)(skb_network_header(skb) +
 				     top_iph->ihl * 4);
 	top_iph->tot_len = htons(skb->len + alen);
-	*(skb_tail_pointer(skb) - 1) = top_iph->protocol;
+	*(skb_tail_pointer(trailer) - 1) = top_iph->protocol;
 
 	/* this is non-NULL only with UDP Encapsulation */
 	if (x->encap) {