[PATCH] swiotlb: support syncing DMA_BIDIRECTIONAL mappings

The current implementation of sync_single in swiotlb.c chokes on
DMA_BIDIRECTIONAL mappings. This patch adds the capability to sync
those mappings, and optimizes other syncs by accounting for the
sync target (i.e. cpu or device) in addition to the DMA direction of
the mapping.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>