ffs_blkatoff_ra
int ffs_blkatoff_ra(struct vnode *, off_t, char **, struct buf **, int);
error = ffs_blkatoff_ra(vp, uio->uio_offset, NULL,
while ((error = ffs_blkatoff_ra(vp, uio->uio_offset, NULL, &bp, 2)) == 0) {