[SK_BUFF]: Use the helpers to get the layer header pointer

Some more cases...

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index b0efd27..4695ada 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -348,7 +348,7 @@
 {
 	struct iphdr *pip = ip_hdr(skb);
 	struct igmphdr *pig = igmp_hdr(skb);
-	const int iplen = skb->tail - skb->nh.raw;
+	const int iplen = skb->tail - skb_network_header(skb);
 	const int igmplen = skb->tail - skb_transport_header(skb);
 
 	pip->tot_len = htons(iplen);