nvme: query namespace identifiers before adding the namespace

Check the namespace identifier list first thing when scanning namespaces.
This keeps the code to query the CSI common between the alloc and validate
path, and helps to structure the code better for multiple command set
support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>