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>

256

@@ -98,7 +98,7 @@ _fill_fs() {
 
 		if [ -f $dir/$file_count.bin ]
 		then
-			bytes_written=`$XFS_IO_PROG -F -c "stat"  $dir/$file_count.bin | grep size | cut -d ' ' -f3`
+			bytes_written=`$XFS_IO_PROG -F -c "stat"  $dir/$file_count.bin | grep stat.size | cut -d ' ' -f3`
 		fi
 
 		# If there was no room to make the file,