md: fix safe_mode buglet.

Whe we set the safe_mode_timeout to a smaller value we trigger a timeout
immediately - otherwise the small value might not be honoured.
However if the previous timeout was 0 meaning "no timeout", we didn't.
This would mean that no timeout happens until the next write completes,
which could be a long time.

Signed-off-by: NeilBrown <[e-mail]>

drivers/md/md.c

···
 		mddev->safemode_delay = (msec*HZ)/1000;
 		if (mddev->safemode_delay == 0)
 			mddev->safemode_delay = 1;
-		if (mddev->safemode_delay < old_delay)
+		if (mddev->safemode_delay < old_delay || old_delay == 0)
 			md_safemode_timeout((unsigned long)mddev);
 	}
 	return len;