rb_tree: optimize rb_insert

In searching for where to insert a new node, RB_INSERT discards the
address of the pointer that will have to be modified, so that it must
find it again from the values of 'parent' and 'comp'. Stop discarding
that address, and so avoid having to recompute it.

Reviewed by: alc
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D36317