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: NGuoqing Jiang <gqjiang@suse.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NSong Liu <songliubraving@fb.com>