Apply a `fixup' to the blocksize gathered from the device after a READ

Apply a `fixup' to the blocksize gathered from the device after a READ
CAPACITY operation.  SCSI-3 mandates this to be 2048, but some older
drives like my old Plasmon CD-R report weird numbers between 2048 and
up to 2352 bytes depending on the mode of the last track etc.  This in
turn confuses stuff like the slice code since it refuses to work with
devices that do not have a blocksize which is a multiple of 512 bytes.

Reviewed by:	ken