scsi: add new scsi-command flag for tagged commands

Currently scsi piggy backs on the block layer to define the concept
of a tagged command.  But we want to be able to have block-level host-wide
tags assigned even for untagged commands like the initial INQUIRY, so add
a new SCSI-level flag for commands that are tagged at the scsi level, so
that even commands without that set can have tags assigned to them.  Note
that this alredy is the case for the blk-mq code path, and this just lets
the old path catch up with it.

We also set this flag based upon sdev->simple_tags instead of the block
queue flag, so that it is entirely independent of the block layer tagging,
and thus always correct even if a driver doesn't use block level tagging
yet.

Also remove the old blk_rq_tagged; it was only used by SCSI drivers, and
removing it forces them to look for the proper replacement.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>

Documentation/block/biodoc.txt

@@ -827,10 +827,6 @@ but in the event of any barrier requests in the tag queue we need to ensure
 that requests are restarted in the order they were queue. This may happen
 if the driver needs to use blk_queue_invalidate_tags().
 
-Tagging also defines a new request flag, REQ_QUEUED. This is set whenever
-a request is currently tagged. You should not use this flag directly,
-blk_rq_tagged(rq) is the portable way to do so.
-
 3.3 I/O Submission
 
 The routine submit_bio() is used to submit a single io. Higher level i/o