scsi: scsi_debug: Simplify request tag decoding

Since commit 64d513ac ("scsi: use host wide tags by default") all
SCSI requests have a tag, whether or not scsi-mq is enabled.

Additionally, it is safe to use blk_mq_unique_tag() and
blk_mq_unique_tag_to_hwq() for legacy SCSI queues. Since this means that
the sdebug_mq_active variable is superfluous, remove it.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>