net/sched: sch_fq: fix integer overflow of "credit"

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

net/sched: sch_fq: fix integer overflow of "credit"

if sch_fq is configured with "initial quantum" having values greater than

INT_MAX, the first assignment of "credit" does signed integer overflow to

a very negative value.

In this situation, the syzkaller script provided by Cristoph triggers the

CPU soft-lockup warning even with few sockets. It's not an infinite loop,

but "credit" wasn't probably meant to be minus 2Gb for each new flow.

Capping "initial quantum" to INT_MAX proved to fix the issue.

v2: validation of "initial quantum" is done in fq_policy, instead of open

coding in fq_change() _ suggested by Jakub Kicinski