idx_encode
if (idx_encode(ctl, idx) == bits)
fp->tag[i] = tag ^ idx_encode(fat->vtw, idx) ^ fatp_xtra[i];
KASSERT(fp->tag[slot] == (tag ^ idx_encode(ctl, idx)
&& fp->tag[slot] == (tag ^ idx_encode(ctl, idx)
KASSERT(fp->tag[slot] == (tag ^ idx_encode(ctl, idx)
&& fp->tag[slot] == (tag ^ idx_encode(ctl, idx)