blk-mq: only apply active queue tag throttling for driver tags

blk-mq: only apply active queue tag throttling for driver tags

If we have a scheduler attached, we have two sets of tags. We don't
want to apply our active queue throttling for the scheduler side
of tags, that only applies to driver tags since that's the resource
we need to dispatch an IO.

Signed-off-by: Jens Axboe <axboe@fb.com>