Reorder manipulations in RB_REMOVE

[dougm]

Summary

For the case when RB_REMOVE requires a nontrivial search to find the node to replace the one being removed, restructure to remove the replacement node, and correct the parent pointers around it first, and let the all-cases code at the end deal with the parent of deleted node, making it point to the replacement node. This removes one or two conditional branches.

[markj]

sys/sys/tree.h

597 ↗

(On Diff #71808)

Perhaps use a local variable for RB_PARENT(old, field)?

[dougm]

Use a local variable for RB_PARENT(old, field).

[pho]

I ran all but the last 7 stress2 tests on D24845.71808.diff without seeing any problems.
I'm now switching to D24845.71955.diff

[pho]

I completed a full stress2 test of D24845.71955.diff. No problems seen except for this known iflib problem: https://people.freebsd.org/~pho/stress/log/dougm070.txt