SPC-3 (rev 13) says that 252 bytes in the maximum (and
recommended) length for a REQUEST SENSE reponse.

Linux asks for 254 bytes in scsi_error.c .

That number was not specified in SPC-2 (although the
allocation length field is 1 byte thus limiting it to
255). Seems as though some numbers are being rounded
down to be multiples of 4.