nvme completion handling refactor and fix v2

[Sagi Grimberg]

> Hi all,
> 
> the first two patches refactor the common nvme completion code
> to be a little less obsfucated, and the third one is the fix
> from Chao to redirect to a different path ported on top of these
> changes, which now make the fix very simple.
> 
> Changes since v1:
>   - add a new patch to handle the path related errors using the
>     SCT class

This looks good to me,

Do you think this is 5.9-rc material? I'm thinking this should go
to 5.10...