[PATCH RFC] nvme: rewrite discard support

[Keith Busch]

On Wed, Mar 16, 2016 at 04:15:32PM -0700, Ming Lin wrote:
> From: Ming Lin <ming.l at ssi.samsung.com>
> 
> This rewrites nvme_setup_discard() similar as sd_setup_discard_cmnd().
> And moves it to common code so fabrics driver can also use it.

Initial thoughts on this, I see the appeal to having a generic NVMe
discard setup, but I liked the existing implementation not needing a
special case for discard check on every IO in the completion handler.

Allocating a page for just 16 bytes of data seems a bit wasteful even
if it is short-lived. When it was allocating 256b from the DMA pool,
the smaller amount of additional data seemed worth it for not creating
a conditional completion.