scsi: target: pscsi: Remove struct pscsi_plugin_task

Copy directly from the se_cmd CDB to the one in the scsi_request.  This
temporarily limits the pscsi backend to supporting only up to 16 byte CDBs,
but this restriction will be lifted later in this series.

Link: https://lore.kernel.org/r/20220224175552.988286-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>