Fix segment/offset calculation. Fix writes that span multiple segments.

[briangreenery]

Thanks for creating this library!

I noticed a problem that not all of the files that I wrote would appear on the floppy drive when I would attach it to a Windows XP virtual machine. When I started debugging this, it looked like if the directory entry write started exactly on a segment, or if a directory entry spanned multiple segments and didn't start at offset 0, then not all of the data would be written.

It looked like this was caused by:

• segment 0, offset 512 (where 512 is the segment size) should be segment 1, offset 0.
• The length of the first buffer written in writeToPosition was computed incorrectly.

[natevw]

Very belated thanks for this patch, I'm reviewing more thoroughly now but in my initial skim looks like the right stuff!

[natevw]

Published as fatfs@0.10.5, thanks again!