rework NVMe command passthrough

[Keith Busch]

On Wed, Nov 09, 2016 at 08:39:56PM +0100, Christoph Hellwig wrote:
> Jens, Keith:
> 
> any opinion on this series?  I'd really prefer to have something
> like this in before merging the NVMe FC support.

Other than the issue you already pointed out in 2/2, this looks good to
me. You can add my review if you just want to make the one fix and push
to the nvme tree.

Reviewed-by: Keith Busch <keith.busch at intel.com>