nvme: export zoned namespaces without Zone Append support read-only

Allow ZNS NVMe SSDs to present a read-only namespace when append is not
supported, instead of rejecting the namespace directly.

This allows (i) the namespace to be used in read-only mode, which is not
a problem as the append command only affects the write path, and (ii) to
use standard management tools such as nvme-cli to choose a different
format or firmware slot that is compatible with the Linux zoned block
device.

Signed-off-by: Javier González <javier.gonz@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>