NVMe: Fix possible queue use after freed

This notifies blk-mq when the tag set contains a different number of
queues prior to freeing unused ones that the request queue points to.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>