nvme: factor out a nvme_ns_is_readonly helper

Add a little helper to check if a namespace should be marked read-only
that uses a new is_readonly flag in the nvme_ns_info structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Javier González <javier.gonz@samsung.com>
Reviewed-by: Joel Granados <j.granados@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>