nvme: cache struct nvme_ctrl reference to struct nvme_request

We will need to reference the controller in the setup and completion
time for tracing and future traffic based keep alive support.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>