Rewrite sectors with read errors

[asomers]

Whenever BFFFS recovers from a faulty read, whether it's EIO or ECKSUM, it should rewrite the faulty sector(s). If the fault device is zoned, then BFFFS must rewrite the entire zone. If not, then it may rewrite just the fault sector(s). Care must be taken not to create a RAID5 write hole in that case.