allow drivers to flush in-flight DMA

Altix supports "posted DMA", so that DMA may complete out of order. In some cases it's necessary for a driver to ensure that in-flight DMA has been flushed to memory for correct operation.

In particular this can be a problem with Infiniband, where writes to Completion Queues can race with DMA of data.

The following patchset addresses this problem by allowing a memory region to be mapped with a "barrier" attribute. (On Altix, writes to memory regions with the barrier attribute have the side effect that in-flight DMA gets flushed to host memory.)

There are three patches in this set:

[1/3] dma: introduce no-op stub "dma_flags_set_dmaflush" [2/3] dma: override "dma_flags_set_dmaflush" for sn-ia64 [3/3] dma: use dma_flags_set_dmaflush in ib_umem_get
(mthca only, for now)

Arthur