simplify the mdev interface v6

[Jason Gunthorpe]

On Tue, Jul 19, 2022 at 10:41:49PM -0400, Eric Farman wrote:

> I suspect the second item (or something similar) is needed anyway,
> because Alex' tree + this series crashes frequently in (usually)
> mdev_remove. I haven't found an explanation for how we get in this
> state, but admittedly didn't spent a lot of time on them since the
> proposed changes to struct subchannel are a non-starter.

It seems strange, at least from a mdev perspective, we shouldn't need
an extra kref on the private.

All references from the mdev universe, including via mdev_remove(),
should halt after mdev_unregister_parent() returns, and ccw calls it
in a context where the private must already be guarenteed valid.

It suggests perhaps mdev_remove() is missing some references?

Jason