fatfs: initialize w/ zeros when seeking past end
fatfs: initialize w/ zeros when seeking past end

The current semantics of fatfs are to not initialize blocks that are
seeked across. This change aligns the seek-past-end semantics with the
standard ones.

fhunleth/fwup,fhunleth/fwup,fhunleth/fwup,michaelkschmidt/fwup,michaelkschmidt/fwup,michaelkschmidt/fwup