rb_tree: fine-tune RB_REMOVE

Improve RB_REMOVE by reading the fields of the removed node only once,
and by not writing to the removed node.

Reviewed by: kib
Discussed with: markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D36288

(cherry picked from commit 5886089ecfc3a7c77bbdf9e95cbf14112f592c50)