dma: pl330: Fix handling of TERMINATE_ALL while processing completed descriptors

The pl330 DMA driver is broken in regard to handling a terminate all request
while it is processing the list of completed descriptors. This is most visible
when calling dmaengine_terminate_all() from within the descriptors callback for
cyclic transfers. In this case the TERMINATE_ALL transfer will clear the
work_list and stop the transfer. But after all callbacks for all completed
descriptors have been handled the descriptors will be re-enqueued into the (now
empty) work_list. So the next time dma_async_issue_pending() is called for the
channel these descriptors will be transferred again which will cause data
corruption. Similar issues can occur if dmaengine_terminate_all() is not called
from within the descriptor callback but runs on a different CPU at the same time
as the completed descriptor list is processed.

This patch introduces a new per channel list which will hold the completed
descriptors. While processing the list the channel's lock will be held to avoid
racing against dmaengine_terminate_all(). The lock will be released when calling
the descriptors callback though. Since the list of completed descriptors might
be modified (e.g. by calling dmaengine_terminate_all() from the callback) we can
not use the normal list iterator macros. Instead we'll need to check for each
loop iteration again if there are still items in the list. The drivers
TERMINATE_ALL implementation is updated to move descriptors from both the
work_list as well the new completed_list back to the descriptor pool. This makes
sure that none of the descripts finds its way back into the work list and also
that we do not call any futher complete callbacks after
dmaengine_terminate_all() has been called.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>