Online filesystem scrubbing and repair

Because bcache works by caching reads from an underlying block device on another device -- usually one caches rotational storage on a device with faster or near-zero seek time, like an SSD. It tries to detect contiguous reads from the underlying device and avoid caching them, but this is not 100% perfect and cannot kick in at once, but only after a few megabytes of reads have been cached, probably pointlessly -- it is better to note that the rotational block device reads are low priority when they are something like a pvmove (which really *is* low priority): these are assumed to be reads for which seek performance does not matter, and thus recent bcache will not cache them.