Restart of ATAPI DMA used to be unreachable, because the request to do
so wasn't indicated in bus->error_status due to the lack of spare bits, and
ide_restart_bh() would return early doing nothing.

This patch makes use of the observation that not all bit combinations were
possible in ->error_status. In particular, IDE_RETRY_READ only made sense
together with IDE_RETRY_DMA or IDE_RETRY_PIO. This allows to re-use
IDE_RETRY_READ alone as an indicator of ATAPI DMA restart request.

To makes things more uniform, ATAPI DMA gets its own value for ->dma_cmd.
As a means against confusion, macros are added to test the state of
->error_status.

The patch fixes the restart of both in-flight and pending ATAPI DMA,
following the scheme similar to that of IDE DMA.

[Including a fixup patch:
Message-id: 1460465594-15777-1-git-send-email-pbutsykin@virtuozzo.com
--js]
Signed-off-by: NPavel Butsykin <pbutsykin@virtuozzo.com>
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Reviewed-by: NRoman Kagan <rkagan@virtuozzo.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Message-id: 1459924806-306-4-git-send-email-den@openvz.org
Signed-off-by: NJohn Snow <jsnow@redhat.com>