scsi: scsi_debug: Simplify command handling

scsi: scsi_debug: Simplify command handling

Simplify command handling by moving struct sdebug_defer into the private
SCSI command data instead of allocating it separately. The only functional
change is that aborting a SCSI command now fails and is retried at a later
time if the completion handler can't be cancelled.

See also commit 1107c7b24ee3 ("scsi: scsi_debug: Dynamically allocate
sdebug_queued_cmd").

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20250224115517.495899-4-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>