vtruncbuf
int vtruncbuf (struct vnode *vp, off_t length, int blksize);
error = vtruncbuf(ovp, length, (int)fs->e2fs_bsize);
error = vtruncbuf(vp, vap->va_size, DEV_BSIZE);
allerror = vtruncbuf(DETOV(dep), length, pmp->pm_bpcluster);