nbuf_ensure_contig
void * nbuf_ensure_contig(nbuf_t *, size_t);
ip = nbuf_ensure_contig(nbuf, sizeof(struct ip));
ip = nbuf_ensure_contig(nbuf, (u_int)(ip->ip_hl << 2));
ip6 = nbuf_ensure_contig(nbuf, sizeof(struct ip6_hdr));
ip6f = nbuf_ensure_contig(nbuf, sizeof(*ip6f));
d = nbuf_ensure_contig(nbuf, ensure);
nptr = nbuf_ensure_contig(&nbuf, sizeof(uint32_t));