NVMe: replace blk_put_request() with blk_mq_free_request()

No point in using blk_put_request(), since we know we are blk-mq.
This only makes sense in core code where we could be dealing with
either legacy or blk-mq drivers. Additionally, use
blk_mq_free_hctx_request() for the request completion fast path,
where we already know the mapping from request to hardware queue.

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