scsi: core: remove Scsi_Cmnd typedef

This will make subsequent refactoring easier to handle.

Note: this patch is nowhere checkpatch clean.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>