256: use more specific grep for file size

256: use more specific grep for file size

Removing the "-F" flag in xfs_io changed stat output.

Before the change when -F was used, whether or not the file
was on xfs, it skipped the extra FSGEOM call.  Now that -F is
removed, it calls FSGEOM if the file is on an xfs fs.  The "size"
grep in test 256 was a bit too loose and matched 2 lines if the
FSGEOM output was present, breaking the test.

A more specific grep for stat.size fixes this.

Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>