cbtorpos
[cbtorpos(&sblock, d)]++;
[cbtorpos(&sblock, d)]++;
[cbtorpos(&sblock, d)]--;
cg_blks(fs, cgp, cylno)[cbtorpos(fs, bno)]--;
cg_blks(fs, cgp, i)[cbtorpos(fs, bno)]++;
cg_blks(fs, cgp, i)[cbtorpos(fs, bbase)]++;