iommu/vt-d: Fix dmar_domain leak in iommu_attach_device

Since commit 1196c2fb a domain is only destroyed in the
notifier path if it is hot-unplugged. This caused a
domain leakage in iommu_attach_device when a driver was
unbound from the device and bound to VFIO. In this case the
device is attached to a new domain and unlinked from the old
domain. At this point nothing points to the old domain
anymore and its memory is leaked.
Fix this by explicitly freeing the old domain in
iommu_attach_domain.

Fixes: 1196c2fb (iommu/vt-d: Fix dmar_domain leak in iommu_attach_device)
Cc: stable@vger.kernel.org # v3.18
Tested-by: Jerry Hoemann <jerry.hoemann@hp.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>