raid5_compute_blocknr
raid5_compute_blocknr(sh, i, 0));
raid5_compute_blocknr(sh, i, 0),
sector_t bn = raid5_compute_blocknr(sh, i, 1);
dev->sector = raid5_compute_blocknr(sh, i, previous);
s = raid5_compute_blocknr(sh, j, 0);
sector_t raid5_compute_blocknr(struct stripe_head *sh, int i, int previous);