iommu/iova: Add rbtree entry helper

Repeating the rb_entry() boilerplate all over the place gets old fast.
Before adding yet more instances, add a little hepler to tidy it up.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/03931d86c0ad71f44b29394e3a8d38bfc32349cd.1614962123.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>