sg_next
new_sg = sg_next(new_sg);
sg = sg_next(sg);
dst = sg_next(dst);
src = sg_next(src);
sg = sg_next(sg);
for (nents = 0; sg; sg = sg_next(sg))
sg = sg_next(sg);
piter->sg = sg_next(piter->sg);
for (_itr = 0, sg = (sglist); _itr < (sgmax); _itr++, sg = sg_next(sg))