zebra: Always receive RS

zebra: The socket is listening on ND_ROUTER_SOLICIT of icmpv6, but won't
recv it when RA is disabled. It will let packets accumulate in kernel.
We should always receive RS packets regardless of whether RA is disabled.

zebra/rtadv.c

@@ -1310,10 +1310,11 @@ void ipv6_nd_suppress_ra_set(struct interface *ifp,
 				zif->rtadv.NumFastReXmitsRemain =
 					RTADV_NUM_FAST_REXMITS;
 			}
-
-			rtadv_start_interface_events(zvrf, zif);
 		}
 	}
+
+	/* Always receive RS, no matter RA is currently disabled */
+	rtadv_start_interface_events(zvrf, zif);
 }
 
 void ipv6_nd_interval_set(struct interface *ifp, uint32_t interval)
@@ -1771,8 +1772,7 @@ void rtadv_if_up(struct zebra_if *zif)
 	 * startup the state machine, if it hasn't been already
 	 * due to a delayed ifindex on startup ordering
 	 */
-	if (zif->rtadv.AdvSendAdvertisements)
-		rtadv_start_interface_events(zvrf, zif);
+	rtadv_start_interface_events(zvrf, zif);
 }
 
 void rtadv_if_init(struct zebra_if *zif)