Revert "block: all blk-mq requests are tagged"

This reverts commit fb3ccb5d.

SCSI-2/SPI actually needs the tagged/untagged flag in the request to
work properly.  Revert this patch and add a follow on to set it in
the right place.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Cc: stable@vger.kernel.org