nvme: split out a nvme_identify_ns_nvm helper

Split the logic to query the Identify Namespace Data Structure, NVM
Command Set into a separate helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>