scsi: sd: Protect against READ(6) or WRITE(6) with zero block transfer length

Since the READ(6) and WRITE(6) commands interpret a zero in the transfer
length field in the CDB as 256 logical blocks, avoid submitting such
commands.

Cc: Douglas Gilbert <dgilbert@interlog.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Reported-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>