[SCSI] scsi_debug: Block Limits VPD page fixes

Add a few clarifying comments in the B0 page function and allow the
optimal transfer length field to be specified on the command line using
opt_blks=N.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>