md: mark md_cluster_mod static

Sparse complains that it has no external declaration, and it turns out
that it is never even used outside of md.c.  So just mark it static
and drop the export.

Acked-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <songliubraving@fb.com>