nvme: enable generic interface (/dev/ngXnY) for unknown command sets

Extend nvme_alloc_ns() and nvme_validate_ns() for unknown command-set as
well. Both are made to use a new helper (nvme_update_ns_info_cs_indep)
which is similar to nvme_update_ns_info but performs fewer operations
to get the generic interface up.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
[hch: rebased on other refactoring patches]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Javier González <javier.gonz@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>