fix data transfer size caculation for special payload requests

[Jens Axboe]

On Fri, Jan 13 2017, Christoph Hellwig wrote:
> Hi all,
> 
> the first two fixes fix a regression in 4.10-rc1 where the data transfer
> size for discard commands wasn't set properly, leading to hangs with
> Hyper-V VMs.  The use the new data transfer size helper added in patch 1
> more widely.

Added for this series, thanks.

-- 
Jens Axboe