dosfs: Ensure initially the sector size is used as bdbuf block size

[Chris Johns]

Ralf Kirchner wrote:
> Hi Ric and all,
> I have added a test case for the problem encountered by Ric.
> I have also created a modified version of Rics patch which is somwhat more conservative.
>
> Patch and test case have been verified with the git HEAD.
>

Ok to commit.

Chris