xlog_cil_iovec_space
alloc_size = nbytes + xlog_cil_iovec_space(niovecs);
memset(lv, 0, xlog_cil_iovec_space(niovecs));
lv->lv_buf = (char *)lv + xlog_cil_iovec_space(niovecs);
xlog_cil_iovec_space(lv->lv_niovecs);