rbtree: Declare augmented callbacks per field with RB_AUG()

RB_DECLARE_CALLBACKS_MULTI() asks its user for a function that copies
the augmented fields and one that recomputes them from the children,
with the early-exit protocol of ->propagate() hand-coded in the latter.
sched/eevdf, its only user, needs five helpers for three fields.

Describe each augmented field instead: RB_AUG(val, aug, fold) names the
per-node value, the member holding the subtree aggregate and how two
aggregates combine (min, max, a sum, ...).  RB_AUG_FUNC() takes a
function for the per-node value.  The template then generates, per
field, a recompute that works on a local and stores once, and a copy,
and combines them into the callbacks; the early exit is the AND of the
per-field results.

RB_DECLARE_CALLBACKS() is the only template left.
RB_DECLARE_CALLBACKS_MAX() becomes RB_AUG_FUNC(RBVALUE, RBAUGMENTED,
max) in a wrapper, with its RBCOMPUTE argument renamed to RBVALUE,
since it returns the per-node scalar and is a different thing from the
RBCOMPUTE of the generic template it used to build on.
RB_DECLARE_CALLBACKS_MULTI() goes away.

sched/eevdf shrinks to a wrapping-safe min() for min_vruntime and
three RB_AUG() lines.

No functional change intended.

[yiwei: take a fold(a, b) instead of a "replace?" compare so that sums
 and counts can be expressed too, which also lets min()/max() replace
 RB_MIN()/RB_MAX(); wrapped the lines over 100 columns]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Yiwei Lin <s921975628@gmail.com>
Assisted-by: LLM
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20260928213736.GA2947991@noisy.programming.kicks-ass.net
Link: https://patch.msgid.link/20261005093236.62702-3-s921975628@gmail.com

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