rb_tree: update augmentation after element change

For an augmented rb_tree, allow a faster alternative to removing an
element from the tree, tweaking it slightly, and inserting it back
into the tree, knowing that its relative position in the tree is
unchanged. Instead, just change the element and invoke
RB_UPDATE_AUGMENT to fix the augmentation data for all the nodes in
the tree.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36010