rejuvenating borderline badblocks

[tomato42]

When scan detects that some blocks are slower than expected, writing to them should improve the situation, either by causing reallocation or improving the strength of the magnetic domain.

Will need to require the scan to work in exclusive mode as it likely would be destructive otherwise.

this is essentially badblocks -n mode, but should be faster as it will apply it just to problematic sectors, not all of them (so about half the time)