blk-mq: let blk_mq_tag_to_rq() take blk_mq_tags as the main parameter

We currently pass in the hardware queue, and get the tags from there.
But from scsi-mq, with a shared tag space, it's a lot more convenient
to pass in the blk_mq_tags instead as the hardware queue isn't always
directly available. So instead of having to re-map to a given
hardware queue from rq->mq_ctx, just pass in the tags structure.

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

diff --cc block/blk-mq.c
Simple merge

diff --cc drivers/block/mtip32xx/mtip32xx.c
Simple merge

diff --cc include/linux/blk-mq.h
Simple merge