vfio: fix build on Linux < 4.2

vfio: fix build on Linux < 4.2

The commit db90b4969e2e ("vfio: retry creating sPAPR DMA window")
introduced a build breakage on old Linux. Linux <4.2 does not define ddw in
struct vfio_iommu_spapr_tce_info. Without ddw, we cannot change window size
and so should give up the creation. I just exculuded the retrying code if
ddw is not supported.

Fixes: db90b4969e2e ("vfio: retry creating sPAPR DMA window")

Signed-off-by: Takeshi Yoshimura <tyos@jp.ibm.com>
Tested-by: Anatoly Burakov <anatoly.burakov@intel.com>