add_out_sign_kvec
add_out_sign_kvec(con, buf, buf_len);
add_out_sign_kvec(con, base, base_len);
add_out_sign_kvec(con, base, crcp - base);
add_out_sign_kvec(con, extdata, extdata_len);
add_out_sign_kvec(con, crcp, CEPH_CRC_LEN);