linuxkpi: Fix DMA_BIDIRECTIONAL mapping

[zishun.yi.dev_gmail.com]

Summary

In dma_sync_single_for_cpu(), the DMA_BIDIRECTIONAL direction currently
performs BUS_DMASYNC_POSTREAD followed by BUS_DMASYNC_PREREAD. This
patch corrects the mapping to use BUS_DMASYNC_POSTREAD |
BUS_DMASYNC_POSTWRITE.

When ownership of the DMA area is transferred to the CPU, we must assume
the previous device access was bidirectional. Both POST operations are
necessary to ensure the CPU sees a consistent view of memory after
potential device reads and writes. A PREREAD is unnecessary here because
the device will no longer access the memory since ownership has been
transferred to the CPU.

Conversely, for dma_sync_single_for_device(), ownership is being
transferred back to the hardware. The buffer must be prepared for
potential bidirectional access by the device, requiring
BUS_DMASYNC_PREREAD | BUS_DMASYNC_PREWRITE.

PR: 293381
Reported by: Zishun Yi <[e-mail]>
Fixes: 95edb10b47fc ("LinuxKPI: implement dma_sync_single_for_*, apply to (un)map single/sg")
Signed-off-by: Zishun Yi <[e-mail]>

[bz]

Isn't this closer to what I once had and hselasky said I should change?
Hah. I came back to this reivew to find the above question unsubmitted. I guess I left it for myself and lost the window.

Bumping this that I may have a look the next days hopefully.