iommu/iova: Simplify domain destruction

All put_iova_domain() should have to worry about is freeing memory - by
that point the domain must no longer be live, so the act of cleaning up
doesn't need to be concurrency-safe or maintain the rbtree in a
self-consistent state. There's no need to waste time with locking or
emptying the rcache magazines, and we can just use the postorder
traversal helper to clear out the remaining rbtree entries in-place.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>