xfs/291: use xfs_repair -f when repairing a fs image

xfs/291: use xfs_repair -f when repairing a fs image

test xfs/291 creates an xfs_metadump image and runs repair on it.
Technically, this is supposed to require "-f" to specify that
the target of the repair is a file; this allows buffered IO fallback
in the case where the image sector size is smaller than the device
sector size.  And in fact if we create the image on a hard 4k device,
the test will fail when repair is unable to operate on the image.

So add that -f!

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>