[IPV4]: When advmss of route is zero, report it as zero not 40.

net/ipv4/route.c

@@ -248,7 +248,8 @@ static int rt_cache_get_info(char *buffer, char **start, off_t offset,
 				r->u.dst.__use,
 				0,
 				(unsigned long)r->rt_src,
-				(int)r->u.dst.advmss + 40,
+				(r->u.dst.advmss ?
+				 (int) r->u.dst.advmss + 40 : 0),
 				r->u.dst.window,
 				(int)((r->u.dst.rtt >> 3) + r->u.dst.rttvar),
 				r->fl.fl4_tos,