checksdstate
enum requeststatus checksdstate(struct sd *sd, struct request *rq, vinum_off_t diskaddr, vinum_off_t diskend);
s = checksdstate(sd, rq, *diskaddr, diskend); /* do we need to change state? */
s = checksdstate(sd, rq, *diskaddr, diskend); /* do we need to change state? */
s = checksdstate(sd, rq, *diskaddr, diskend); /* do we need to change state? */