scsi: core: Move the result field from struct scsi_request to struct scsi_cmnd

Prepare for removing the scsi_request structure by moving the result field
to struct scsi_cmnd.

Link: https://lore.kernel.org/r/20220224175552.988286-7-hch@lst.de
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>