[SK_BUFF]: Introduce skb_network_offset()

For the quite common 'skb->nh.raw - skb->data' sequence.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

security/selinux/hooks.c

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d41e24d..addb585 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2944,7 +2944,7 @@
 	int offset, ihlen, ret = -EINVAL;
 	struct iphdr _iph, *ih;
 
-	offset = skb->nh.raw - skb->data;
+	offset = skb_network_offset(skb);
 	ih = skb_header_pointer(skb, offset, sizeof(_iph), &_iph);
 	if (ih == NULL)
 		goto out;
@@ -3026,7 +3026,7 @@
 	int ret = -EINVAL, offset;
 	struct ipv6hdr _ipv6h, *ip6;
 
-	offset = skb->nh.raw - skb->data;
+	offset = skb_network_offset(skb);
 	ip6 = skb_header_pointer(skb, offset, sizeof(_ipv6h), &_ipv6h);
 	if (ip6 == NULL)
 		goto out;