CVE-2026-23168: flex_proportions: make fprop_new_period() hardirq safe

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

flexproportions: make fpropnewperiod() hardirq safe

Bernd has reported a lockdep splat from flexible proportions code that is essentially complaining about the following race:

<timer fires> runtimersoftirq - we are in softirq context calltimerfn writeoutperiod fpropnewperiod writeseqcountbegin(&p->sequence);

<hardirq is raised> ... blkmqendrequest() blkupdaterequest() ext4endbio() folioendwriteback() wbwriteoutadd() fpropaddpercpumax() if (unlikely(maxfrac < FPROPFRACBASE)) { fpropfractionpercpu() seq = readseqcountbegin(&p->sequence); - sees odd sequence so loops indefinitely

Note that a deadlock like this is only possible if the bdi has configured maximum fraction of writeout throughput which is very rare in general but frequent for example for FUSE bdis. To fix this problem we have to make sure write section of the sequence counter is irqsafe.