scsi: scsi_transport_sas: check reply payload length instead of bidi request

As a user of bsg-lib the SAS transport should not poke into request
internals but use the bsg_job fields instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>