xfs: skip stale inodes in xfs_iflush_cluster

We don't write back stale inodes so we should skip them in
xfs_iflush_cluster, too.

cc: <stable@vger.kernel.org> # 3.10.x-
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>