nvme: enable char device per namespace

[Christoph Hellwig]

On Tue, Jan 12, 2021 at 07:30:55PM +0100, Javier González wrote:
> I have not implemented multipath support, but it should definitely not
> crash like this. I'll rebase to 5.11 and test.
>
> Christoph: Is it OK to send this without multipath or should I just send
> all together?

We're not in a rush at the moment, so I'd prefer working multipath
support if you can spare a few cycles.