md/raid5: switch to use conf->chunk_sectors in place of mddev->chunk_sectors where possible
The chunk_sectors and new_chunk_sectors fields of mddev can be changed
any time (via sysfs) that the reconfig mutex can be taken. So raid5
keeps internal copies in 'conf' which are stable except for a short
locked moment when reshape stops/starts.
So any access that does not hold reconfig_mutex should use the 'conf'
values, not the 'mddev' values.
Several don't.
This could result in corruption if new values were written at awkward
times.