scsi: log request tag for scmd_printk()

The request tag provides a concise identification of a SCSI
command, so we should be printing that out for scmd_printk().

Suggested-by: Christoph Hellwig <hch@lst.de>
Tested-by: Robert Elliott <elliott@hp.com>
Reviewed-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>