sched/fair: Fix overflow in vruntime_eligible()

Zhan Xusheng reported running into sporadic a s64 mult overflow in
vruntime_eligible().

When constructing a worst case scenario:

If you have cgroups, then you can have an entity of weight 2 (per
calc_group_shares()), and its vlag should then be bounded by: (slice+TICK_NSEC)
* NICE_0_LOAD, which is around 44 bits as per the comment on entity_key().

The other extreme is 100*NICE_0_LOAD, thus you get:

{key, weight}[] := {
  puny: { (slice + TICK_NSEC) * NICE_0_LOAD, 2               },
  max:  { 0,                                 100*NICE_0_LOAD },
}

The avg_vruntime() would end up being very close to 0 (which is
zero_vruntime), so no real help making that more accurate.

vruntime_eligible(puny) ends up with:

 avg  = 2 * puny.key (+ 0)
 load = 2 + 100 * NICE_0_LOAD

 avg >= puny.key * load

And that is: (slice + TICK_NSEC) * NICE_0_LOAD * NICE_0_LOAD * 100, which will
overflow s64.

Zhan suggested using __builtin_mul_overflow(), however after staring at
compiler output for various architectures using godbolt, it seems that using an
__int128 multiplication often results in better code.

Specifically, a number of architectures already compute the __int128 product to
determine the overflow. Eg. arm64 already has the 'smulh' instruction used. By
explicitly doing an __int128 multiply, it will emit the 'mul; smulh' pattern,
which modern cores can fuse (armv8-a clang-22.1.0). x86_64 has less branches
(no OF handling).

Since Linux has ARCH_SUPPORTS_INT128 to gate __int128 usage, also provide the
__builtin_mul_overflow() variant as a fallback.

[peterz: Changelog and __int128 bits]
Fixes: 556146ce5e94 ("sched/fair: Avoid overflow in enqueue_entity()")
Reported-by: Zhan Xusheng <zhanxusheng1024@gmail.com>
Closes: https://patch.msgid.link/20260415145742.10359-1-zhanxusheng%40xiaomi.com
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260505103155.GN3102924%40noisy.programming.kicks-ass.net

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