net: Correct comparisons and calculations using skb->tail and skb-transport_header

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 whereas skb->transport_header
will be an offset from head. This is corrected by using wrappers that
ensure that comparisons and calculations are always made using pointers.

Signed-off-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/inet_ecn.h

···
 {
 	switch (skb->protocol) {
 	case cpu_to_be16(ETH_P_IP):
-		if (skb->network_header + sizeof(struct iphdr) <= skb->tail)
+		if (skb_network_header(skb) + sizeof(struct iphdr) <=
+		    skb_tail_pointer(skb))
 			return IP_ECN_set_ce(ip_hdr(skb));
 		break;
 
 	case cpu_to_be16(ETH_P_IPV6):
-		if (skb->network_header + sizeof(struct ipv6hdr) <= skb->tail)
+		if (skb_network_header(skb) + sizeof(struct ipv6hdr) <=
+		    skb_tail_pointer(skb))
 			return IP6_ECN_set_ce(ipv6_hdr(skb));
 		break;
 	}

net/core/dev.c

···
 			skb_reset_mac_header(skb2);
 
 			if (skb_network_header(skb2) < skb2->data ||
-			    skb2->network_header > skb2->tail) {
+			    skb_network_header(skb2) > skb_tail_pointer(skb2)) {
 				net_crit_ratelimited("protocol %04x is buggy, dev %s\n",
 						     ntohs(skb2->protocol),
 						     dev->name);
···
 	NAPI_GRO_CB(skb)->frag0 = NULL;
 	NAPI_GRO_CB(skb)->frag0_len = 0;
 
-	if (skb->mac_header == skb->tail &&
+	if (skb_mac_header(skb) == skb_tail_pointer(skb) &&
 	    pinfo->nr_frags &&
 	    !PageHighMem(skb_frag_page(frag0))) {
 		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);