sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf()

It was possible to have pick_eevdf() return NULL, which then causes a
NULL-deref. This turned out to be due to entity_eligible() returning
falsely negative because of a s64 multiplcation overflow.

Specifically, reweight_eevdf() computes the vlag without considering
the limit placed upon vlag as update_entity_lag() does, and then the
scaling multiplication (remember that weight is 20bit fixed point) can
overflow. This then leads to the new vruntime being weird which then
causes the above entity_eligible() to go side-ways and claim nothing
is eligible.

Thus limit the range of vlag accordingly.

All this was quite rare, but fatal when it does happen.

Closes: https://lore.kernel.org/all/[email protected]/
Closes: https://lore.kernel.org/all/CA+9S74ih+45M_2TPUY_mPPVDhNvyYfy1J1ftSix+KjiTVxg8nw@mail.gmail.com/
Closes: https://lore.kernel.org/lkml/[email protected]/
Fixes: eab03c23c2a1 ("sched/eevdf: Fix vruntime adjustment on reweight")
Reported-by: Sergei Trofimovich <[email protected]>
Reported-by: Igor Raits <[email protected]>
Reported-by: Breno Leitao <[email protected]>
Reported-by: kernel test robot <[email protected]>
Reported-by: Yujie Liu <[email protected]>
Signed-off-by: Xuewen Yan <[email protected]>
Reviewed-and-tested-by: Chen Yu <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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