sched/fair: Fix overflow in update_tg_cfs_runnable()

In the Linux kernel, the following vulnerability has been resolved:

sched/fair: Fix overflow in update_tg_cfs_runnable()

A divide-by-zero crash is observed when running hackbench:

[14697.488452] CPU: 112 UID: 0 PID: 124791 Comm: hackbench Not tainted 7.1.0-rc2+
[14697.492627] RIP: 0010:propagate_entity_load_avg+0x35f/0x3e0
[14697.506799] <TASK>
[14697.507411] __dequeue_task+0x2b4/0xc70
[14697.508677] dequeue_task_fair+0x36/0x370
[14697.509047] dequeue_task+0x101/0x2f0
[14697.509426] __schedule+0x1b1/0x1a00
[14697.510868] anon_pipe_read+0x3da/0x450
[14697.511400] vfs_read+0x361/0x390
[14697.512053] __x64_sys_read+0x19/0x30

The divide-by-zero happens here:

if (scale_load_down(gcfs_rq->load.weight)) {
load_sum = div_u64(gcfs_rq->avg.load_sum,
scale_load_down(gcfs_rq->load.weight));
}

gcfs_rq->load.weight is an insane large value and is truncated
to the lower 32 bits by div_u64, which happen to be 0.

Using AI for investigation, the cause is a u32 overflow in
update_tg_cfs_runnable(), and flat pickup became a victim when using
tg_tasks():

u32 new_sum, divider;
...
new_sum = se->avg.runnable_avg * divider; <-- boom

The following sequence shows how this triggers the crash:

propagate_entity_load_avg()
update_tg_cfs_runnable() # u32 overflow corrupts runnable_sum

__update_load_avg_cfs_rq()
___update_load_avg() # computes insane runnable_avg
update_tg_load_avg() # propagates to tg->runnable_avg

update_cfs_group()
calc_concur_shares()
tg_tasks() # long-to-int truncation, negative nr
reweight_entity() # corrupted se->load.weight
update_load_add() # corrupted cfs_rq->load.weight

propagate_entity_load_avg()
update_tg_cfs_load()
div_u64() # divide-by-zero

Fix by widening new_sum from u32 to u64 (no need to force tg_tasks()
to return unsigned long after this fix)