ipr crashes due to NULL dma_need_drain since cc97923a5bcc ("block: move dma drain handling to scsi")

[Christoph Hellwig]

On Tue, Jun 09, 2020 at 08:00:35PM +1000, Michael Ellerman wrote:
> Hi all,
> 
> I'm seeing crashes on powerpc with the ipr driver, which I'm fairly sure
> are due to dma_need_drain being NULL.

Ooops, my changes completely forgot about SAS attached ATAPI devices.
I'll cook up a fix in a bit.