md: minor cleanup in safe_delay_store.

There isn't really much room for races with ->safemode_delay.
But as I am trying to clean up any racy code and will soon
be removing reconfig_mutex protection from most _store()
functions:
 - only set mddev->safemode_delay once, to ensure no code
   can see an intermediate value
 - use safemode_timer to call md_safemode_timeout() rather than
   calling it directly, to ensure it never races with itself.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -3242,11 +3242,13 @@ safe_delay_store(struct mddev *mddev, const char *cbuf, size_t len)
 		mddev->safemode_delay = 0;
 	else {
 		unsigned long old_delay = mddev->safemode_delay;
-		mddev->safemode_delay = (msec*HZ)/1000;
-		if (mddev->safemode_delay == 0)
-			mddev->safemode_delay = 1;
-		if (mddev->safemode_delay < old_delay || old_delay == 0)
-			md_safemode_timeout((unsigned long)mddev);
+		unsigned long new_delay = (msec*HZ)/1000;
+
+		if (new_delay == 0)
+			new_delay = 1;
+		mddev->safemode_delay = new_delay;
+		if (new_delay < old_delay || old_delay == 0)
+			mod_timer(&mddev->safemode_timer, jiffies+1);
 	}
 	return len;
 }