mkfs: don't warn about topology when mkfsing a file

mkfs: don't warn about topology when mkfsing a file

Some tests were failing for me like:

 QA output created by 206
 === truncate file ===
 === mkfs.xfs ===
+warning: unable to probe device toplology for device /mnt/test/fsfile.21615
...

I thought about filtering the tests, but ...

It seems that we shouldn't even be trying to get topology for a
plain file, should we?  And then we won't warn about failure.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>