nvme: set the chunk size before freezing the queue

We don't need a frozen queue to update the chunk_size, which just is a
hint, and moving it a little earlier will allow for some better code
reuse with the multipath code.

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