scsi: target: Allow userspace to set the completion type

scsi: target: Allow userspace to set the completion type

This allows userspace to request if we complete in the backend context
or the frontend driver. It works the same as submission where you can
write 0 to 2 to complete_type:

 0 - Use the fabric driver's preference.

 1 - Complete from the backend calling context if the fabric supports
     it.

 2 - Queue the completion to LIO's completion workqueue.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://patch.msgid.link/20260222232946.7637-4-michael.christie@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>