[IP]: Introduce ip_hdrlen()

For the common sequence "skb->nh.iph->ihl * 4", removing a good number of open
coded skb->nh.iph uses, now to go after the rest...

Just out of curiosity, here are the idioms found to get the same result:

skb->nh.iph->ihl << 2
skb->nh.iph->ihl<<2
skb->nh.iph->ihl * 4
skb->nh.iph->ihl*4
(skb->nh.iph)->ihl * sizeof(u32)

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index aba3ff0..54b7543 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -539,7 +539,7 @@
 static int ipmr_cache_report(struct sk_buff *pkt, vifi_t vifi, int assert)
 {
 	struct sk_buff *skb;
-	int ihl = pkt->nh.iph->ihl<<2;
+	const int ihl = ip_hdrlen(pkt);
 	struct igmphdr *igmp;
 	struct igmpmsg *msg;
 	int ret;