flex_proportions: make fprop_new_period() hardirq safe

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

flex_proportions: make fprop_new_period() hardirq safe

Bernd has reported a lockdep splat from flexible proportions code that is

essentially complaining about the following race:

<timer fires>

run_timer_softirq - we are in softirq context

call_timer_fn

writeout_period

fprop_new_period

write_seqcount_begin(&p->sequence);

<hardirq is raised>

...

blk_mq_end_request()

blk_update_request()

ext4_end_bio()

folio_end_writeback()

__wb_writeout_add()

__fprop_add_percpu_max()

if (unlikely(max_frac < FPROP_FRAC_BASE)) {

fprop_fraction_percpu()

seq = read_seqcount_begin(&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.