md: make sure manual changes to recovery checkpoint are saved.

If you make an array bigger but suppress resync of the new region with
  mdadm --grow /dev/mdX --size=max --assume-clean

then stop the array before anything is written to it, the effect of
the "--assume-clean" is lost and the array will resync the new space
when restarted.
So ensure that we update the metadata in the case.
Reported-by: NSebastian Riemer <sebastian.riemer@profitbricks.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -3782,6 +3782,8 @@ resync_start_store(struct mddev *mddev, const char *buf, size_t len)
 		return -EINVAL;
 
 	mddev->recovery_cp = n;
+	if (mddev->pers)
+		set_bit(MD_CHANGE_CLEAN, &mddev->flags);
 	return len;
 }
 static struct md_sysfs_entry md_resync_start =