nvme: introduce NVME_NS_METADATA_SUPPORTED flag

This is a preparation for adding support for metadata in fabric
controllers. New flag will imply that NVMe namespace supports getting
metadata that was originally generated by host's block layer.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>