net, ipv4, ipv6: Correct assignment of skb->network_header to skb->tail

This corrects an regression introduced by "net: Use 16bits for *_headers
fields of struct skbuff" when NET_SKBUFF_DATA_USES_OFFSET is not set. In
that case skb->tail will be a pointer however skb->network_header is now
an offset.

This patch corrects the problem by adding a wrapper to return skb tail as
an offset regardless of the value of NET_SKBUFF_DATA_USES_OFFSET. It seems
that skb->tail that this offset may be more than 64k and some care has been
taken to treat such cases as an error.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index 795498f..d2ede89 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -2642,6 +2642,7 @@
 	__be16 *svlan_tci = NULL;                /* Encapsulates priority and SVLAN ID */
 	__be16 *svlan_encapsulated_proto = NULL; /* packet type ID field (or len) for SVLAN tag */
 	u16 queue_map;
+	unsigned long tail_offset;
 
 	if (pkt_dev->nr_labels)
 		protocol = htons(ETH_P_MPLS_UC);
@@ -2708,7 +2709,12 @@
 		*vlan_encapsulated_proto = htons(ETH_P_IP);
 	}
 
-	skb->network_header = skb->tail;
+	tail_offset = skb_tail_offset(skb);
+	if (tail_offset > 0xffff) {
+		kfree_skb(skb);
+		return NULL;
+	}
+	skb_set_network_header(skb, tail_offset);
 	skb->transport_header = skb->network_header + sizeof(struct iphdr);
 	skb_put(skb, sizeof(struct iphdr) + sizeof(struct udphdr));
 	skb_set_queue_mapping(skb, queue_map);
@@ -2775,6 +2781,7 @@
 	__be16 *svlan_tci = NULL;                /* Encapsulates priority and SVLAN ID */
 	__be16 *svlan_encapsulated_proto = NULL; /* packet type ID field (or len) for SVLAN tag */
 	u16 queue_map;
+	unsigned long tail_offset;
 
 	if (pkt_dev->nr_labels)
 		protocol = htons(ETH_P_MPLS_UC);
@@ -2822,7 +2829,12 @@
 		*vlan_encapsulated_proto = htons(ETH_P_IPV6);
 	}
 
-	skb->network_header = skb->tail;
+	tail_offset = skb_tail_offset(skb);
+	if (tail_offset > 0xffff) {
+		kfree_skb(skb);
+		return NULL;
+	}
+	skb_set_network_header(skb, tail_offset);
 	skb->transport_header = skb->network_header + sizeof(struct ipv6hdr);
 	skb_put(skb, sizeof(struct ipv6hdr) + sizeof(struct udphdr));
 	skb_set_queue_mapping(skb, queue_map);