vxlanudphdr
ifp->if_hdrlen = ETHER_HDR_LEN + sizeof(struct vxlanudphdr);
struct vxlanudphdr *hdr;
MPASS(len >= sizeof(struct vxlanudphdr));
M_PREPEND(m, sizeof(struct ip) + sizeof(struct vxlanudphdr),
M_PREPEND(m, sizeof(struct ip6_hdr) + sizeof(struct vxlanudphdr),