CDRIOCSETBLOCKSIZE accepts block_size=0 -> divide-by-zero kernel panic

Summary

CDRIOCSETBLOCKSIZE stores user-supplied int directly into cdp->block_size with no validation then immediately calls acd_set_ioparm which computes rounddown(cdp->iomax cdp->block_size) macro divides by second argument. block_size=0 triggers integer divide-by-zero trap in kernel context. Any operator-group user single ioctl deterministic panic no disc required. O_RDONLY sufficient no FWRITE check on ioctl.