netinet: provide "at offset" variant of the in_delayed_cksum() API

The need for such a variant comes from the fact that we need to
re-calculate checksum aftet ng_nat(4) transformations while getting
mbufs from the layer 2 (ethernet) directly.

Reviewed by:	markj, tuexen
Approved by:	tuexen
Sponsored by:	Sippy Software, Inc.
Differential Revision:	https://reviews.freebsd.org/D49677
MFC After:	2 weeks

sys/netinet/ip_output.c

@@ -1044,14 +1044,14 @@ ip_fragment(struct ip *ip, struct mbuf **m_frag, int mtu,
 }
 
 void
-in_delayed_cksum(struct mbuf *m)
+in_delayed_cksum_o(struct mbuf *m, uint16_t iph_offset)
 {
 	struct ip *ip;
 	struct udphdr *uh;
 	uint16_t cklen, csum, offset;
 
-	ip = mtod(m, struct ip *);
-	offset = ip->ip_hl << 2 ;
+	ip = (struct ip *)mtodo(m, iph_offset);
+	offset = iph_offset + (ip->ip_hl << 2);
 
 	if (m->m_pkthdr.csum_flags & CSUM_UDP) {
 		/* if udp header is not in the first mbuf copy udplen */
@@ -1078,6 +1078,13 @@ in_delayed_cksum(struct mbuf *m)
 		*(u_short *)mtodo(m, offset) = csum;
 }
 
+void
+in_delayed_cksum(struct mbuf *m)
+{
+
+	in_delayed_cksum_o(m, 0);
+}
+
 /*
  * IP socket option processing.
  */

sys/netinet/ip_var.h

@@ -271,6 +271,7 @@ VNET_DECLARE(struct pfil_head *, inet_local_pfil_head);
 #define	PFIL_INET_LOCAL_NAME	"inet-local"
 
 void	in_delayed_cksum(struct mbuf *m);
+void	in_delayed_cksum_o(struct mbuf *m, uint16_t o);
 
 /* Hooks for ipfw, dummynet, divert etc. Most are declared in raw_ip.c */
 /*