rb_tree: optimize tree rotation

The RB_ROTATE macros begin with fetching a field via a pointer. In
most cases, that value is one that has already been pulled into a
register, and the compiler cannot infer that. So, to eliminate those
needless fetches, have the caller of the RB_ROTATE macros present the
data in the third macro parameter, rather than having the macro fetch
it.

Differential Revision: https://reviews.freebsd.org/D35520