scsi: core: Use scsi_cmd_priv() instead of open-coding it

Improve code readability without modifying the behavior of the code.

Cc: Hannes Reinecke <hare@suse.de>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20250624210541.512910-4-bvanassche@acm.org


Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>