scsi: core: Remove struct scsi_request

Let submitters initialize the scmd->allowed field directly instead of
indirecting through struct scsi_request and remove the now superfluous
structure.

Link: https://lore.kernel.org/r/20220224175552.988286-8-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>