md-cluster: make function cluster_check_sync_size static

The function cluster_check_sync_size is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'cluster_check_sync_size' was not declared. Should it be static?
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/md-cluster.c

@@ -1094,7 +1094,7 @@ static void metadata_update_cancel(struct mddev *mddev)
 /*
  * return 0 if all the bitmaps have the same sync_size
  */
-int cluster_check_sync_size(struct mddev *mddev)
+static int cluster_check_sync_size(struct mddev *mddev)
 {
 	int i, rv;
 	bitmap_super_t *sb;