vt-d: fix the IOMMU flush issue

vt-d: fix the IOMMU flush issue

The propagation value from IOMMU flush interfaces may be positive, which
indicates callers need to flush cache, not one of faliures.

when the propagation value is positive, this patch fixes this flush issue
as follows:
  - call iommu_flush_write_buffer() to flush cache.
  - return zero.

Signed-off-by: Quan Xu <quan.xu@intel.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>