nvme: refactor ns info helpers

nvme: refactor ns info helpers

Pass in the nvme_ns_head pointer directly. This reduces the necessity on
the caller side have the nvme_ns data structure present. Thus we can
refactor the caller side in the next step as well.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>