ng_l2cap_prepend
m = ng_l2cap_prepend(m, sizeof(ng_l2cap_clt_hdr_t));
con->rx_pkt = ng_l2cap_prepend(con->rx_pkt, sizeof(*hdr));
m0 = ng_l2cap_prepend(m0, sizeof(*l2cap_hdr));
m0 = ng_l2cap_prepend(m0, sizeof(*acl_hdr));
struct mbuf * ng_l2cap_prepend (struct mbuf *, int);