nvme: simplify error logic in nvme_validate_ns()

We only should remove namespaces when we get fatal error back from
the device or when the namespace IDs have changed.
So instead of painfully masking out error numbers which might indicate
that the error should be ignored we could use an NVME status code
to indicated when the namespace should be removed.
That simplifies the final logic and makes it less error-prone.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>