dat_split_crste
rc = dat_split_crste(mc, *last, gfn, asce, uses_skeys);
rc = dat_split_crste(mc, *last, gfn, asce, uses_skeys);
rc = dat_split_crste(mc, *last, gfn, asce, uses_skeys);
rc = dat_split_crste(mc, *last, gfn, asce, uses_skeys);
return dat_split_crste(p->mc, crstep, gfn, walk->asce, false);