iommu_gas: avoid pointless augmentation

iommu_gas: avoid pointless augmentation

iommu_gas_augment_entry updates a map entry element. Invoked as
RB_AUGMENT in RB tree code, it is applied from the point where the
tree is modified, all the way up to the root, and is also applied when
rotation moves a node down in the tree.

There are several opportunities to invoke it less. The automatic
augmentation with every rotation is a mistake.  Delaying these
augmentations until RB_INSERT_COLOR or RB_REMOVE_COLOR are finishing
allows the augmentation code to be duplicated less, to work when there
is less register pressure, and to be skipped when conditions allow it:

    In the double-rotate case of RB_INSERT_COLOR, augmentation after
    the first rotation is not necessary when the element being moved
    down the tree becomes a leaf. It was in the tree, and was a leaf,
    before the RB_INSERT operation began, and so recomputing
    augmentation for it would do nothing.

    In the final (possibly only) rotation of RB_REMOVE_COLOR, both the
    elements - the one moving up and the one moving down - end up in
    the path from the deletion point to the tree root, so there's no
    need to augment either of them immediately.

    In RB_REMOVE, when the right child of the removed node replaces it
    in tree, it began with a null left child. Replacement creates a
    non-NULL left child, and then rotation may put a NULL node back in
    that place. If that happens, start the augmenting-up-to-root with
    the parent of that node, since augmentation would do nothing.

Adjust to avoid these needless augmentations.

Reviewed by:	alc
MFC after:	3 weeks
Differential Revision:	https://reviews.freebsd.org/D35502