nvme: properly free resources for cancelled command

We need to move freeing of resources to the ->complete handler to ensure
they are also freed when we cancel the command.

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