iplink: add missing link type

Add missing link type "bridge_slave".
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>

ip/iplink.c

@@ -93,7 +93,7 @@ void iplink_usage(void)
 		fprintf(stderr, "TYPE := { vlan | veth | vcan | dummy | ifb | macvlan | macvtap |\n");
 		fprintf(stderr, "          bridge | bond | ipoib | ip6tnl | ipip | sit | vxlan |\n");
 		fprintf(stderr, "          gre | gretap | ip6gre | ip6gretap | vti | nlmon |\n");
-		fprintf(stderr, "          bond_slave | ipvlan | geneve }\n");
+		fprintf(stderr, "          bond_slave | ipvlan | geneve | bridge_slave }\n");
 	}
 	exit(-1);
 }