[RFC PATCHv4 2/3] nvme: introduce multipath_always_on module param

[Christoph Hellwig]

I just applied this locally, but ran into somethig that I think is
not addressed here.  NVMe allows private namespaces to have non-unіque
nsids if certain features (like ANA) are not enabled.  I don't think
we handle this here, and in fact we can't really support that with
the multipath node.  But we'll at least need to reject that case
gracefully somehow and document it.