xfstests 042: point xfs_fsr right at the fragmented file

xfstests 042: point xfs_fsr right at the fragmented file

Somehow 042 as it stands isn't actually defragging the fragmented
file we created (see 042.full post-run)

Probably need to look into that as well, but for now this should
make the call to fsr actually _do_ something.

This has let 2 fsr bugs slip through qa in the past few months.   :(

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@infradead.org>