Receive buffers need to be mapped with DMA_FROM_DEVICE.  Incorrectly
mapping with DMA_TO_DEVICE causes a hard lock on ppc64 machines with
an IOMMU.

This fixes <https://bugs.openfabrics.org/show_bug.cgi?id=431>
Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>