nvmet: export I/O characteristics attributes in Identify

Make the NVMe NAWUN, NAWUPF, NACWU, NPWG, NPWA, NPDG and NOWS attributes
available to initator systems for the block backend.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>