iommu/vt-d: Use list_for_each_entry_safe() for dmar_domain->devices traversal

Replace list_for_each_safe() + list_entry() with the simpler
list_for_each_entry_safe().

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>