sched/fair: Avoid overflow in enqueue_entity()

sched/fair: Avoid overflow in enqueue_entity()

Here is one scenario which was triggered when running:

    stress-ng --yield=32 -t 10000000s&
    while true; do perf bench sched messaging -p -t -l 100000 -g 16; done

on a 256CPUs machine after about an hour into the run:

    __enqeue_entity: entity_key(-141245081754) weight(90891264) overflow_mul(5608800059305154560) vlag(57498) delayed?(0)
    cfs_rq: zero_vruntime(3809707759657809) sum_w_vruntime(0) sum_weight(0) nr_queued(1)
    cfs_rq->curr: entity_key(0) vruntime(3809707759657809) deadline(3809723966988476) weight(37)

The above comes from __enqueue_entity() after a place_entity(). Breaking
this down:

    vlag_initial = 57498
    vlag = (57498 * (37 + 90891264)) / 37 = 141,245,081,754

    vruntime = 3809707759657809 - 141245081754 = 3,809,566,514,576,055
    entity_key(se, cfs_rq) = -141,245,081,754

Now, multiplying the entity_key with its own weight results to
5,608,800,059,305,154,560 (same as what overflow_mul() suggests) but
in Python, without overflow, this would be: -1,2837,944,014,404,397,056

Avoid the overflow (without doing the division for avg_vruntime()), by moving
zero_vruntime to the new entity when it is heavier.

Fixes: 4823725d9d1d ("sched/fair: Increase weight bits for avg_vruntime")
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
[peterz: suggested 'weight > load' condition]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260407120052.GG3738010@noisy.programming.kicks-ass.net