nvme-auth: check chap ctrl_key once constructed

ctrl ctrl_key member may be overwritten from a sysfs context driven
by the user. Once a queue local copy was created, use that instead
to minimize checks on a shared resource.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>