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

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

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.