[SCSI] scsi_debug: Fix incorrect page length in logical block provisioning VPD

The page length for the 0xb2 VPD page is defined to be 4 bytes when no
provisioning descriptors are provided (DP=0).

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>