Add compat support for DMA_ATTR_SKIP_CPU_SYNC

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
backports/v3.1_no_dma_buf_h.patch: fix dma_buf_export prototype

dma_buf_export has an extra argument these days, so update the
dummy function.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>